I think it's instructive to look at this from the other side to see why these processes exist. At my last job, I wanted to hire a frontend engineer with some experience beyond tweaking HTML and CSS (we used gRPC/Web and Typescript, so some actual programming experience was going to make them pretty productive, and that's what I was looking for).
Anyway, I wrote up a req and posted it to the usual career sites, and within a day, I had 500 resumes to review. About 490 of these had no real-world programming experience. They were all candidates from "boot camps", that, as far as I can tell, copied some example code into their GitHub and called it a day. (They listed each example project as "work experience", and many of the candidates had bit-for-bit identical repositories in their GitHub.) I would have been fooled if I had only seen one of these, but when you see hundreds of them all on the same day, you realize that investing in each candidate isn't possible. Even a 30 minute phone call to each candidate would have cost me an entire month of 8 hour workdays. Even a cursory review ("maybe this person actually did something other than the boot camp, let's look through GitHub") was excessively time-consuming. Even an email along the lines of, "sorry, we're looking for 5 years of experience for this senior position", is expensive.
So at that point, you have to somehow shift the cost onto candidates. It costs nothing to spam your resume to hundreds of companies, and I have no doubts that the bootcamps automate this process to some extent. That is why online coding tests exist. You have so many candidates in the pipeline that missing one good person is a cost that's acceptable for having to spend 30 minutes with someone that probably can't actually program. Is it impersonal? Yes. Does it suck for someone to be in limbo for forever because you are too lazy to write them an email saying "we're not moving forward with the process." Yes. But the submitting-resumes process scaled and was automated much more efficiently than the reviewing process. It's kind of like spam filtering now. (I get a ton of emails in my spam folder from people from HN that are email-blasting everyone on HN their projects. I get a ton of emails in my spam folder from people in open-source Slack channels that I read along the same lines. Is it unfortunate that I can't read and respond to every one of these emails? Yes. Does it suck that their "hustle" got detected by some machine-learning algorithm that has permanently blacklisted their company. Yes. But that is life -- attention is limited, a for loop that sends email isn't. So something has to give.)
I realize that it's confusing to say "we can't hire any engineers!" and at the same time throw 500 resumes into the trash can. But a resume is not equal to a hirable engineer. Anyone can make a resume. And, attracted to the high salaries of the software engineering field, many people do!
Anyway, I don't know what other fields are like, but I feel like the programming world is unique. You don't need extensive background -- university, internships, prior work experience. You can buy yourself a Python book and teach yourself everything you need to work at Google all by yourself. As a result, there are a lot of people that think they've taught themselves everything they need to work at Google all by themselves, but in fact can't actually do the job. As a hiring manager, your job is to filter the wheat from the chaff, and there is one grain of wheat in every ton of chaff. It's hard, and you're going to miss something. But, that is the world we built for ourselves. In other fields, there are other bodies in place that handle the filtering ("what's your Professional Engineer license number?"), but not in ours. As a result, the hiring process isn't as good.
I think we could do better, and I do appreciate people that I've emailed and wrote back "the process is over". But even that is hard to scale.
Internships seem super valuable to me. When I was at Google I had an intern every summer, and I basically dedicated my summer to teaching them everything they needed to know to be successful at Google or another big software company. Obviously, Google was 100% happy about this; I would go to weekly team meetings, say "all I did last week was spend days with $INTERN figuring out how to adjust build options for this new target" and the response was not "wow, you're lazy, get back to work" but "great!". Not every company can support that, though, which is why they're looking for people with experience. They want to pay money for getting work done, not pay money to have a senior engineer do no work for 3 months with the hope of someday having someone who can get things done by themselves. Again, it sucks, but that's how it is. Job training is EXPENSIVE. You can't always expect to get it for free. (That's why colleges exist, not that I think they do a great job.)
TL;DR: Practicing those coding challenges and how to write A* on the whiteboard is going to send a strong signal to companies that you're worth their time. It shouldn't be that way, but it is. "The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man." You have to ask yourself, do you want a job, or do you want to change the world. Changing the world is going to be a lot harder. Kind of sucks, but that's life.