Because grabbing a random prefix from the pool is easier than remembering which prefix is assigned to which subscriber account and keeping it static through ISP network changes.
My ISP does 'sticky' prefixes, which means they change when they move users between BNGs but otherwise don't.
I know whenever I try to help someone with a Linux issue it's always an uphill battle to get them to stop guessing what they think the problem might be and show me the logs.
People really don't want to give you the information you need to help them.
It also calls all the customers who don't want to be locked out of the product they paid for 'fickle' and brought up gamergate out of nowhere, so I'm honestly not sure what sort of agenda they're pushing.
Because grabbing a random prefix from the pool is easier than remembering which prefix is assigned to which subscriber account and keeping it static through ISP network changes.
My ISP does 'sticky' prefixes, which means they change when they move users between BNGs but otherwise don't.