If you actually try to do this yourself, you’ll learn why Bootstrap is popular. You’ll spend plenty of time making everything consistent across browsers, responsive, visually consistent, accessible (a legal requirement for most of us), etc. If you’re working on a big project with a dedicated team, that approach can give you quite a lot but there are a ton of projects which prefer to spend their time on the core product.

That’s a legitimate decision responding to their environment – you might disagree because you have different goals and resource levels but it’s wortb understanding why not everyone makes the same call.