Do you think mechanics misdiagnose the cars problem to make more money?

Not at all. If they really don't know what the problem is, then you will get your car back and it will not be fixed. If they tell you something is wrong, and its really ok, then that is another story. That happens plenty, and most people are oblivious and just hand over their credit card or a wad of cash.

Its all about the upsell. "Well if I only fix this, you'll probably be back in here in a few months, but if I fix this too, you'll be good to go for a long time" :cha-ching:

Its up to the consumer to not be taken, unfortunately.

I don't think this happens nearly as much as when they say they are providing a service, but don't. Ex: oil change, tire rotation, fluid flushes or top offs, electronic diagnostic testing, emissions testing, most points on the inspection list...

Talk to a mechanic and ask for thier most ridiculous stories, you'd be surprised.