> building your own UI library

It's one more thing to maintain, and it's also difficult to push back on things. If you use off the shelf components it's much easier to say to designers and managers that a UX pattern is not available or not valid. You can point to the mature well known community owned UI library you use and make it authoritative. It's harder to do it if you build your own, suddenly each designer and developer is throwing things in there, adding features etc. It's also difficult to agree on the structure, so the components are well thought out, flexible, but also not so flexible to lose semantics. It's not an easy job, do you use slots, composition, rendering callbacks, there are too many decisions and you spend time building the UI library instead of actually shipping features.