1. they think it makes newer devs can feel safer
2. dev teams can simply agree on an pre-made modus-operandi for all css without having to have discussions/evolutions on it.
But for 1, it also gives newer devs a steeper learning curve, and it gives older devs Yet Another Framework to learn and then possibly discard in a few years. It's also often used like a crutch, where yes the newer dev may feel safer, but they're also not exposed to as many discussions or reasoning around well-organized styling, cleaner semantics, etc.
And for 2, it can also add overhead to dependency audits and often for the large frameworks with pre-made components you can end up spending significant resources fighting/working around the framework components when the inevitable unforeseen use-cases come up