If you NEED to buy a car for under $5000 you don't have any extra money laying around. You need to spend that money on food, or insurance, or rent, or any number of other things. Yes a PPI is a great idea and yes the tech deserves fair pay for his time, but if you're poor and your last car just fell apart you're trying to get back on the road as cheap as possible. If you pay $50 (or however much it costs for a good inspection) each for 10 cars in a row on Craigslist, now you can't afford to actually buy any of them. So you don't get any of them inspected and you buy one that's not leaking anything (because every fluid in the car is 100% stop-leak of some kind) and then 3 days later you have a problem and then you end up having your car posted here.
Yes, you can do research yourself and try to learn what to look for, but poverty and bad education are tied together pretty tightly. If you're poor you're probably not very good at self-directed study and you're not going to get very far. The whole situation sucks. This is why I try to help everyone that I know avoid mistakes with car buying and maintenance.