"Why do people always advise others to find a new job before quitting the current one?"

There are a lot of jobs out there but the interview process can take months and after being exhausted from searching, you may well just accept a bad deal.

I've been in bad jobs though, and tried to job hunt from there. There was one where there's a meeting every half day to discuss progress. It was so draining that it was a confidence hit during interviews. I quit and it felt like the days were literally more beautiful.