Why did the original series do dumb shit like referring to levels as if they were something that actually existed and Pokemon learning in a specific order?

This is entirely false, as Pikachu is still learning new moves, and Pokemon often learn moves out of order (Froakie learning Double Team instead of Quick Attack) or learn moves that are TMs or Tutors in the games naturally (Pikachu learning Electro Ball, and then Electro Ball upgrading into Electroweb).

There's a lot of shit you can chalk up to early installment weirdness, but referring to living creatures in terms of numbers was always a retarded thing to do.