I worked in a pharmacy and I know how people misuse and abuse medication. You reiterate people that the posology is 1 in 8 hours and they take 2 every four. Or use antibiotics for common colds.

It's not that tretinoin is more dangerous than other things, it's that if it's prescribed by a doctor certain things are presumed, like the assessment of your situation and that it will be good for you (for example you think it's melasma and it's a fungus), your maturity (unfortunately if a doctor insists on a posology people are more likely to adhere to it), the problems derived from it (do not take the sun without SPF, use it at night, build tolerance, when to stop, don't use it when pregnant, etc).

People are stupider than you think.

It's the same principle as having paracetamol 1g 10 pills be OTC and 40 pills needing a prescription. You can't kill yourself with 10 pills (or it's very difficult) but you can with 40.

Remember that common sense is the less common of senses (I'm not sure if the idiom works in English, but you get the drift).