Why is only the ? and ! put at the front and upside down of a sentence in Spanish?

I would assume it's because it leads the reader to what tone to use in a given sentence. The question mark or exclamation point would be useful in tone throughout the whole sentence, but if neither is present in front of the sentence a regular reading tone could be assumed.

so why add a floating period when nothing being there allows for the same assumption and is much, much simpler and easier?

Still learning Spanish but I believe this is correct, because you can insert a question mark into the middle of a sentence as well if the entire sentence isn’t a question.
Ex:
I have fish, do you want to cook it?
Tengo pescado, ¿quieres cocinarlo?
- You can do that and it's grammatically correct? :O
As a Spaniard: yes.
- *si

It’s because Spanish sentence structure is different from English. In Spanish the sentences “Can I tell you? (¿Te lo puedo decir?) and “I can tell you.” (Te lo puedo decir.) are formed the same way. The initial punctuation lets the reader know that the sentence is a question or exclamation or not so they can parse the sentence properly from the start.

Sentence structure likely plays a role but, at the end of the day, it's just a spelling convention - people do it because they do it. And it's generally absent from the standard orthography of Portuguese and Italian, even if they're syntactically similar to Spanish (i.e. no German/English-like VSO for questions).
- Yeah, it's just tradition at this point, though I feel like native speakers really try to oversell its usefulness when someone questions if the opening signs are necessary. People act like they routinely need to read text written like the Cartas de relación out loud, and thus, need the additional warning lest they get lost in the long, multi-clause sentences. Like, I could understand if you had to read something like
  Y después acá, por no haber oportunidad, así por falta de navíos y estar yo ocupado en la conquista y pacificación de esta tierra, como por no haber sabido de la dicha nao y procuradores, no he tornado a relatar a vuestra majestad lo que después se ha hecho; de que Dios sabe la pena que he tenido. Porque he deseado que vuestra alteza supiese las cosas de esta tierra, que son tantas y tales que, como ya en la otra relación escribí se puede intitular de nuevo emperador de ella, y con título y no menos mérito que el de Alemaña, que por la gracia de Dios vuestra sacra majestad posee. Y porque querer de todas las cosas de estas partes y nuevos reinos de vuestra alteza decir todas las particularidades y cosas que en ellas hay y decir se debían, sería casi proceder a infinito.
  out loud on a regular basis, but even contemporary literary Spanish doesn't tend to have nearly the same amount of sentences that just go one for half a page, much less the sort of stuff people would write to each other normally.
  As you've mentioned, other syntactically similar languages do just fine without them, even including other Romance languages spoken in various regions of Spain. The only exception I'm aware of is Asturianu, which apparently also uses them, though apparently they're optionally allowed in Galego Real Academia Galega. On page 38 of the PDF, it says they're entirely optional if you want to facilitate reading by including them.
Well now I have to wonder why other languages don't do the front punctuation thing. It would be just as helpful in English as it is in Spanish to know a sentence is a question before getting to the end of it. Scrambling to lift your voice into a question you've been reading as a statement until you see the question mark at the end is awkward. lol
- Scrambling to lift your voice into a question you've been reading as a statement until you see the question mark at the end is awkward
  I'd argue this is exactly why it's not necessary on English; what makes it a question tends to be the inflection at the very end so no real need for a warning way at the beginning.
English manages the exact same thing without the leading punctuation.
- English changes the order of the words of the sentence. Spanish uses punctuation marks. It’s just differences in languages. Personally I appreciate them since it helps me read Spanish quicker with fewer parsing errors.
- English, you mean that notoriously easy to parse and learn language that has rules so obscure that native speakers don't even realize they're adhering to them? Tell that to my Irish old small lovely mother.
  https://dictionary.cambridge.org/grammar/british-grammar/adjectives-order

The ¿ and ¡ prepare the reader mentally for what's coming and let the speaker adapt pronunciation.

Consider the following 2 sentences in English:

It's raining.

and

It's raining?

Meaning and intonation are different. Luckily our eyes don't read strictly in one direction like a scanner but instead they skip back and forth a lot (saccades) which means your brain registers the question mark even before you get to pronounce the first word. Still it's helpful to have an extra signal at the start of the sentence.

So why no extra dot at the beginning? Because it's the default case. And since the function of the dot is to separate sentences a single one already does the job. Note how there is also no double period when a sentence ends with an abbreviation or abbr. And in headers it's often fully omitted because the layout itself signals the separation from what precedes or follows.

Here's a period: .

Here's an upside-down period: .

Hope that clears things up.

Period would be the default, so no reason to include it as well. Questions and exclamations have a different tone than simple statements, so it's more useful. It's particularly useful in Spanish, as the word order/conjugation is changed less frequently than in English, so it's going to help a reader understand intent more quickly. It seems like it became established as "proper" in 1754 by Spain's Royal Academy. It also would have been very easy for printers, as they would just chuck their existing type into the tray upside down.

Fun fact but it's actually a pretty recent thing for the language in the first place. While first done in the 1700's iirc, there are Spanish language publications that didn't follow the punctuation rules as recently as the 1900's.