They have similar effects, yes, but the point of access, and the usage, differ. When a caster is gaining access to 3rd level spells, a paladin is only then gaining access to 2nd level spells. When a cleric is gaining access to 5th level spells, a paladin is merely gaining access to 3rd level spells. When a cleric is hitting their cap of 9th level spells, a paladin is merely gaining access to 5th level spells. So while, yes, a 5th level spell is pretty much a 5th level spell, you have one group of classes casting 5th level spells when they reach ninth level, and another group of classes casting 5th level spells when they reach seventeenth level. That's quite a dramatic spread of when they're flinging these respective spells about, and that is an important mechanical interaction to take in to account. So, rather than making one spell X level on the cleric list, and Y level on the paladin list, they simply limit when paladins gain access to spells of X level across the board. Same basic idea, much simpler implementation.