I know why they're doing this and IMHO it's a dead end. Among companies with tight budgets(I worked for a few of those) there's a notion that you'll get more value out of someone who already knows the required framework so you should focus on that in the recruiting process. That may be true for small, short-lived projects(3 months max.), but what matters the most in the larger, longer ones is something, that can only be expressed as the ability to program.
Basically it's a sign that this company is mainly occupied with doing the grunt work for larger companies. A job like that sure, pays your bills, but is also soul-crushingly boring.
> Along with that, it seems like most web dev jobs just are hooking up API endpoints for applications that don't get a lot of traffic, so there are no interesting speed/performance problems to solve.
Well, this is web development for you. Hell, most of the time this is software engineering for you.
But there's a way out of this: Like I said before - a company with its own product that does something fundamental(like e.g. a framework). I spent two years in such a company and I can tell you this - I only dealt with interesting problems there.