Because the ability to just suck it up & get things done even when you don't enjoy every minute of it is a very important trait to have. It's not just about the tests.
> "Why do I care about whether this series converges?"
You can abstract anything to the point of irrelevance. Don't. Try to learn something from it. You're developing problem-solving skills, not merely finding the sum of a series.
I think that this is true, but after four years of working hard at a demanding high school I was actually substantially less able to "suck it up and get things done." I don't know if that's because high school burnt me out or because I changed mentally during high school or something else, but it makes me a lot less willing to accept that forcing people to work hard on boring shit makes them more likely to work hard on other boring shit in the future.
I've run into this a lot doing data analysis. Sometimes it takes 20-30 minutes to manually clean up the data (or more often, if it's structured enough, write a script to do it), but other people often overestimate the amount of work required, and decide that the grunt work isn't worth it. But it lets me make better conclusions from the data, so it's usually worth the effort.
An enviable skill is to be able to see past the grunt work to the value down the road.
There are sometimes when you have to do work that you don't like and you just have to suck it up and do it, but it's much easier and rewarding when this job is useful (example, researching a lot to finally decide which type of structure you'll incorporate with.
I'm saying that a good way for teachers to engage students more is to show them examples that actually have some reflection on the real world, so that they can't say "this is totally useless. I won't bother learning it."