I think it comes down to one of two things; the first is ignorance. If the only tool you have is a hammer, everything starts to look like a nail. If you only know about React and Sass, any layer beneath that is a complete mystery and not something you'd even consider as a solution because you're not aware of it.

The second, and my preferred theory, is that the modern web stack has not adequately followed increasing user requirements, forcing us to take much longer to develop what seems like trivial new features. It's not that i18n, or larger images or maintaining state in the browser is some huge, insurmountable problem that can't be solved. It's just that our tools are so woefully immature that we spend more time wrestling with them than we do actually working on the features we're being paid to work on.

And for what? For some vague illusion of choice that for the vast majority of people doesn't even matter? I don't care if I use Redux or one of the bazillion different variations on the idea. Ditto for Sass, Tailwind or what have you. The web layer needs to solidify already, give us a decent bedrock to build our apps and websites on and just get out of the way.