So why not design the game around thirty levels? How about forty levels?
Or just open-ended. Indeed, why have levels at all?
I think, d20 has 20 levels, because symmetry, makes about as much sense as D&D has 20 levels because 2e had 20-level tables. That is, it's arbitrary, and doesn't really matter. (What might be said to matter, if, indeed, anything about mechanics does, is what the presented level range covers, the whole '0 to hero' thing, with the understanding that 'hero' isn't the same for every class, and that the fighter's gonna be linear and the full-casters quadratic,
or else edition warring, of course.)
Why have spells be every odd level? Why not have ten spell levels that are spread across ten levels? That seems easier to understand for newcomers.
It would be. Much. But it wouldn't be familiar enough to the old-timers, who would therefore view it as "too complicated for new players," and tell them so, all over the internet.
I likely betrayed my bias in my earlier post. I lean more towards ten levels.
13A went with that, it is more intuitive. And, it covers about the same tiers as 5e, in concept, and your character grow more by the numbers, since it's also using the more intuitive +1/1 leveling bonus, rather than 5e's 2+(level/5) or 4e's level/2.