As a back end developer who needs to do bits and pieces of front end stuff, I can see this taking me months (at least) to learn JavaScript CSS, and design to make something as anywhere close to bootstrap. Or I could just use what they have done (and thousands have tested) and save myself a whole lot of effort. And if you have other developers joining the team, at least they will be familiar with whatever framework you are using, rather than some in house crap that someone wrote ages ago, and has now left the company.
I have not looked at the source code for Bootstrap, but if it was crap I assume there would be a ton of articles ranting about like there are for PHP.
So (with Bootstrap at least) I am getting pre-written, tested, understood code, documented code, probably of at least "reasonable" quality, with a community online to help with problems. Does you own code have that? I certainly don't have time to write that stuff myself .
The more experienced I get as a developer, the less code I try to write. 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.
simple enough once you've worked with CSS long enough
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.
IMO, if I were to arrive on a new project and saw the front-end devs reinventing the wheel, it would immediately set off a red flag.
That may be your problem. Maybe I'm a glutton for punishment, but I especially enjoy this type of work.
CSS frameworks like Bootstrap provides you with a tested baseline, so you can be confident that the front-end you created will look the same across a wide range of browsers.
Certainly Bootstrap won't be suitable for all scenarios, but starting from a framework is generally a good idea.
Many websites have a whole lot of overlap in terms of UI patterns (a nav bar, responsive grids, etc). Is it really necessary for every dev to hand-code that nav bar, or fight with CSS's terrifyingly bad grid support? Not really, which is why these frameworks are nice.
Would you elaborate what you mean by fighting? Bootstrap is extremely simple to customize and I am no front-end guru.
A specific example: Bootstrap expects dropdowns to be unordered lists with anchors as the top-level element, so having a fixed-position header as the first item while its children scroll is difficult. If this has been our CSS based on our markup, it would've taken fifteen minutes, because we could have gone in and done a bit of basic surgery on our module, making it more flexible. (This may not be the best example, but that's the general idea.)