I've built a team from trained-up junior devs, and it was great, I would recommend this route for anyone building a team. BUT... you have to have people who can train and teach as well as code. There are personality traits that don't align with this. The typical "aspy Dave" stereotype of a rockstar programmer is not good at this. So you have to align the entire team from day 1 as being a training environment as well…
From the very start, we put a lot of effort in our hiring and onboarding process. We treat hiring as the beginning of onboarding.
Oh, and we were too small to have an HR department. When we grew big enough, they definitely got in the way. I had to more or less fight them off over every position we hired for, because they didn't understand anything about coding, IT or creativity, and would just look for relevant experience in a similar field.
Our company is smallish with a startup vibe but recently we hired a VP of HR with a lot of corporate experience and it has started going this direction, too.
And there I think you have the answer to the OP's question: companies cargo-culting their hiring process and strategy and succumbing to that tendency that Daniel Kahneman identifies as the root of all bias: WYSIATI.