And for a thought experiment - if a lawyer controls the pace of their work, why work at any reasonable speed? They could take the client's money and go picnic or something then put in 20 minutes at the end of the day.
Something is forcing them to put in long hours. It's the market.
That’s a supply and demand issue. If there were more clients than lawyers, I can totally see a reality where they speed things up because now in one week they can work on 20 cases instead of 2, thus 10x more money, but as you said, it doesn't seem to be the reality of the market.
edit: tldr; it does not seem in their best interests to be more efficient at this point
Because they have ethics rules to only bill for hours actually worked. If they did that, they would only get paid for 20 minutes of work.