First off, I disdain the idea of infantizing people as having zero impulse control. Lots of people every day understand the concept of restraint and control. When I buy a tub of ice cream I don't eat the whole thing in one frenzy of chocolate fudgy goodness. I know how to portion control. I don't need a wise leader telling me "That's enough" and only hand me a single scoop. Lots of people know how to budget their paycheck, drive the speed limit, and bite their tongue in public. Adults do it all the time. Yes, some people DO have problems with self-restraint, control or addiction. These are considered disorders to be treated professionally. But the vast majority of people can, when addressed in a rational manner, understand the reasons for limitation and restraint.
As an experiment, I once ran a one-shot game years ago in 3.5 where the ability score generation method was "give yourself what you think your character would have; I trust you." No dice, no points. Pure honor system. Did anyone give themselves all 18's? They could have, but no one did. They opted for a high score in their primary, a decently high in a secondary, and a smattering of medium or so in everything else. Later, I looked at the ability scores players gave themselves and reverse engineered Point Buy cost. The average was 38 points, slightly higher than normal point buy. Nobody took advantage of the 'I win" button. Everyone created characters you could have rolled if your dice were great but not exceptional.
Why? By your hypothesis, the players should have given themselves all maxed scores barring a strong rule to create limited (PB) or randomized (dice) scores. Did they yield to some sort of unwritten social pressure? (Not wanting to appear too greedy or munchkin)? Did they do that because they felt bad about "cheating" and opted to tone down their choices? Or maybe, they felt they wanted to be good at their primary function but have some weakness or areas not well defined. Maybe they felt their character should not be super smart, strong, or charismatic.
Then again, it was a sample size of six players I already knew and played with. Maybe 100 players would have yielded far higher ability scores, even the proverbial all 18s.
So I don't accept your hypothesis that players if given a chance will act like toddlers only interested in their own pleasure. Most players and not pure id. Some people will, but some people will cheat at dice rolls, lie to the DM, and engage all manner of bad sportmanship. That's just life.
In our very first 5E game, we look at point buy and decided the ability scores didn't give enough power for the type of game the DM wanted. So we decided to use the old point buy from 3.5 with the heroic 32 point buy where you can have up to an 18 before racial mods at level 1. So I built a PC with nothing below a 10.
After the campaign was over we agreed to never do that again. The PCs felt too much like Mary/Marty Stus with no flaws to play off of. It's a myth with no backing that the majority of people just want overpowered PCs. Those people certainly exist, but it's a small percentage of the general population and I suspect even for many of those it would quickly get boring.