Now you too are only explaining why people like to roll.
Again:
Why build D&D like a casino, where the best option is not to roll?
I question the assertion that, with regards to hp, D&D was built like a casino. The casino option is there, but the assumed option, and the only option for organized play, is not to roll for hit points.
If rolling for hit points were statistically better than average, more players would want to roll for hp so as to get the best outcome for their characters. As it is, the system discourages rolling for hp, so the only players who will do it are those for whom the act of rolling the dice is, in of itself, a reward.
I believe that rolling for hp adds very little to the game, unless you just like more random and arbitrary in your life. (Roll for how many slices of pizza you get, and then roll for how much you pay me.)
As for losing hp on leveling up — if you have a negative con modifier and are rolling for hp, I think there is a part of you that
wants the possibility of a bad outcome. That's the fun of it. Playing stupid in D&D isn't fun if it sometimes doesn't come crashing down. So hell yeah, that frail individual is getting sickly, spending too much time inside with their books and their Great Old Ones.
---EDITED TO ADD-----
I would warn the player with low constitution before they rolled, of course, that this is a possible outcome, and, if they prefer, they can just take the average. I would never force a player to roll for hp in 5e. This isn't B/X.