OK, and where do I start? How do I make a pretty blue border around a text input to subtly indicate focus? What's the best gradient / padding to use in such circumstances? What fade in time will looks good? There are so many questions I could ask just about the text input highlighting.The thing is, if you actually designed web UIs professionally and didn't rely on these toolkits, you could immediately make a reasonable first guess at all of those things and you'd be doing it in the context of your specific project. The total time to refine everything over the entire development cycle might be a few minutes, and most of that would be cross-browser testing, which obviously for professional work you should be doing even if you're using a toolkit.
I've nothing against toolkits. They can be useful for quick prototypes. They do a competent basic job for in-house tools written by people who aren't front-end developers and just need something that works. But for high-end UI work? I have never written a major professional project that still used any of these toolkits in the final production code.
Most toolkits aren't sufficiently reliable that you can trust them without testing, so you've still got one of the biggest and most tedious jobs to do whether you use one or not.
Often attention to detail is lacking in significant areas. Using a big name toolkit is no guarantee of quality; many of them have absolute howlers on their own demo pages that are immediately obvious in at least one major browser, often just in the visuals before you even start interacting.
But more than any of that, for real projects, you're often going to have branding requirements that will mean extensive customisation. All those default colour and typography schemes aren't worth much. And some all-things-to-all-people toolkit that works great if you use its defaults might be hiding all kinds of limitations if you're not using a flat design, or if your layout requirements are more demanding than "simple responsive grid", or if you want something that works right on mobile or supports proper keyboard navigation or meets accessibility requirements or...
Toolkits are great for basic work, and for a lot of projects that really is all you need, but you don't design a Rolls Royce by picking wheels and bodywork from a catalogue.
People with the "code everything yourself" attitude remind me of when I was fresh out of university, and didn't realise how incompetent I was at the time.
Code everything myself? Unlikely. Code things myself when quality matters and off-the-shelf isn't good enough? Always.