ELI5 How the WPA under Rosevelt now considered socialism? It employed millions of people and they built some of the most remarkable things. How come this is frowned upon in the US?

True answer: no one was squeezing the project for profit.

That’s it. Rich people don’t want anything to happen here they can’t grab a piece of. Fucking equity gooners

But they could, and did.

It's nothing new. The WPA was created in the era of political machines and the Red Scare. Early on, opponents worried that Roosevelt was creating a nationwide political machine run by far-left radicals.

It’s frowned upon now because it benefitted people who weren’t already extremely wealthy

It was socialism.

It wasn't. But it should have been.
What it was was the government doing stuff.

For people on the Right. FDR is the devil himself. Look up NAM (National Association of Manufacturers)

So I don't know if the WPA itself would be considered socialism. I imagine it depends on your working definition of socialism.

What I can say with a little more confidence is that the WPA and a bunch of other New Deal programs were a direct result of pressure from American Socialists and Communists at the time.

I can't recall if this is apocryphal or not, but the story goes that FDR told a private gathering of America's biggest capitalists about his plans for the New Deal and told them they were going to pay for it through taxes. When they protested for a compromise, FDR said "this is the compromise. The alternative is guillotines." But I can't find that quote anywhere, so I may have made it up.

That quote may not be true, but American's were certainly desperate enough. Having heard how mom and dad grew up in the Depression, and young adulthood in WWII, I laugh at GenZ thinking they have it bad.
"No. You have it bad compared to your parents and grandparents. Whine to their parents and let's see how that plays out."
- I sympathize with Gen Z. It's pretty natural to see your parents' generation having done way better than you are doing at the same age and feel like you've had something stolen from you. It's a pretty profound sense of loss, esp. when you do look at history and think, "Jesus Christ, do we have to suffer like the Great Depression to turn this shit around?"

Why would being a good thing make it not socialism?

It's not?

Counterpoint
No.
The WPA was the government doing stuff.
Socialism is the workers owning the means of production.