Because you never know what’s going to happen at the new job. You might be given circumstances you cannot adhere to or deal with people you simply can’t put up with.

Starting a new job is always a gamble and can fail easily

If you quit because of bad conditions, like unsafe conditions.

If you get a new job and put in two weeks, your first job might immediately fire you. Teo weeks without pay is a crisis for a lot of people.

If you need to spend time applying and interviewing you might not be able to work your crappy job during that time.

These are US issues for low income earners who are afraid of getting a new job.