As I understand it 3-18 was understood to be the adult human range with 18/00 being the maximum. Other races had ranges more or less within this, but they were different. Beyond 18/00, for any ability, meant one was buffing up their racial stats, were magically changed into another race/monster type, or were wearing magic items, etc. Why was it 3-18/00 for everyone at the table, no matter the race chosen? That had to do with the theory of the game at the time.
I like that they are pushing racial ability modifiers. I like that they are thinking about rolling stat increases into classes too. Both have been done before, with the second more recent than the first, but it allows stats to increase because of gaining a level. That's something that was paid for differently as I understand it. For whatever reason, class abilities increased separately from core abilities - the big 6 (and more, but let's stick to the point). The latter could be boosted through training, but it took a lot of time and the absence of adventuring. Seeing how magic was an easier path, that's what usually won out.
The main point though is this: 3-18 defines a nice spectrum of what stories are going to be told by the game. We're not talking about flea circuses (literally) or Galactus the devourer of worlds. The abilities scores define the scope of the game to readily recognizable behavior by the participants engaging in it. Could a STR 128 eat a planet? Why not. Could 18? Sure. How about 0.0003? Yeah, that too. The thing about the statistics is they define a somewhat comprehensible fictional world that isn't so different from our own that we cannot relate.
Does this mean we should make PCs with ability scores in the 100s? Millions? Infinities? Go right ahead. The core game is going to be focusing on commonly understood human scales. We can build from there. With that reliability we can design everything else in the game to balance with that spectrum of scores. Then we are told how to break them. Or we break them ourselves and drop maximums.