Basically, getting a job for a senior developer is the easiest thing ever to require so much work. You're in hot market demand, but if you spread your search out to include multiple companies, the hours of screenings, interviews, paperwork, and coding challenges start to really add up. Clearly that's frustrating.
For managers, hiring a bad fit is simply disastrous. Turning around and firing them is never easy. Yeah, it's slow, and that person drags your team while it happens. If you can make it fast, that has its own affect on your team culture. I understand this is an unsympathetic observation, but: firing people is emotionally hard on managers. So, yeah, we'd rather pick right the first time than play a "fire fast" iterated numbers game.
I'd propose that the existence of the test isn't the problem, it's all the stuff around it. As a hiring manager, my job is to ask you to do this thing you don't want to do, and to explain why it's important for the company and the team. This is the first time, but not the last time we'll have a conversation like that. You should note whether or not I'm good at it and whether you'll tolerate that kind of thing from me. Because as an employed engineer you'll end up tolerating those requests from someone.
As a candidate, you can stand out by making a deal. "Hey, I'm really excited about this opportunity. But I have several companies in my process plus I'm still employed at my current job. I understand your side—a resume doesn't always tell you if someone can code, and I definitely don't want to work with other people who can't pass something like this. Still, I need to manage my time well, and this is too much. Is there any way we could cut the size of the challenge in half? Or, I've got an open bug on an open source project I contribute to. Can I work on that, put in a PR, then discuss that in the interview? Trying to get us both what we need here. Thanks."
Personally, I'd respond really well to a discussion like that, and I think all my hiring mangers would. And the discussion itself would make me want to hire you more: you're a team player, you're transparent, you aren't dismissive of me but you'll advocate for yourself.