|Any sufficiently advanced|
maths is indistinguishable
However, the other way of looking at the question "why does it take a 5th level spell to create a moonbeam" is "nobody knows". That is to say that the published spells are a qualitative description of how magic works. It's easy to put people to sleep, but for some inexplicable reason it's hard to make an ordinary arrow burst into flame. It's very hard to teleport without risk; it's quite easy to speak to animals.
It's inconsistent, but that's the nature of magic, or anything which is not actually well understood and has to be treated as a black box. From outside the box, the rules produce surprising results which are often unpredictable. Perhaps somewhere there's a class which finds flame arrow really simple; perhaps not. Perhaps the moon god had some sort of argument with the god of magic. Perhaps the goddess of sleep married the god of magic.
Or perhaps magic is weird and unpredictable and spell casters have to cope with that as best they can.
It's not like sleep being first level is really any odder how an electron works, is it?