the default assumption is that scores are rolled.
This was, IMO, one of the worst design decisions in the entire edition. It's a concept that is foreign to most gamers nowadays, and while it does something unique, that thing it does is only welcome in a very uncommon play style.
Rolling stats works if you are playing the game as a survival challenge, of if you
don't already have a character concept. Most people already have a character concept before they roll stats. And most people don't have an urge to take what the dice give them and challenge themselves to make it survive and thrive.
No, most people come to the game wanting to play a tough, strong dwarf (or nowadays, more like a wood elf monk or a tiefling warlock, but I digress), possibly with a name, and a backstory. But if you are rolling for stats (and/or hp), you are just as likely to be unpleasantly surprised by low rolls that interfere with your concept as you are with high rolls that empower it. And if your entire character concept is a tough dwarf, but you end up with a Con 14, and roll a lot of 1s and 2s for hit points, while the halfling rogue or elf cleric in the party ended up with a Con 16 and keeps rolling 7s and 8s for hit points, your entire character concept is shot.
Sure, you could pretend you're a tough dwarf who for some reason doesn't have the mechanics to back it up. Or you could change your concept--but that's the whole problem. You can't come with a concept and expect to be able to play it with rolled stats.
And if you make exceptions to let people reroll if the rolls aren't good enough for their concept, you still have the issue of objective imbalance amongst PCs in an objectively balanceable area. Does anyone actually find that particular thing fun? I mean really? That's straight up a no go for me. Even when I'm playing a game where we are rolling for stats, I make sure to homebrew a method where the stats are objectively equal for each character. (For instance, everyone randomly rolls stats, but then everyone can pick to use whichever rolled array they like--including everyone using the same one if desired.) If PCs having an objective imbalance in one of the few areas in D&D where objective balance is actually possible is something a DM wants to do, they need to recognize that is what they want to do and make it completely transparent to their players to see if they are on-board.