>>58303212Well firstly you can change abilities now, Ability Capsules have let you swap between a Pokemon's main abilities since I think Gen 6? And Ability Patches were introduced in Gen 8 I believe, which let you swap between main abilities and hidden abilities. So full customization there is actually already achieved!
As for why every Pokemon (unless you're Calyrex) only has one ability at a time from a maximum of three possibilities, it's most likely to keep competitive play more balanced. When each Pokemon only has a handful of options to choose from, and can only use one at a time, it's a lot easier to strategize around them and figure out how to deal with them. It's already possible to get blindsided by someone running a Pokemon with only two abilities who opts to use the less popular one, so imagine the chaos of trying to plan for top threats that could have any one of five abilities? Or multiple different combinations of those? Keeping the cap at three just keeps things a lot more manageable. (And let's be honest, it's also probably a lot easier to code)
If you're interested in what it would look like if Pokemon could have multiple abilities at once, I know there's a format on Showdown that's based around the idea of every Pokemon having all their available abilities at once. Like Gastrodon has Storm Drain, Sticky Hold, and Sand Force all at the same time, for instance. Maybe you could look into that.