Again, the comparison isn't even in the same league. Going from d6 to d8 only means upping your max possible damage by 33% of the original. In the other case, we are talking about an increase of almost 100% of the damage.

Also, about attack damage, ranged touch attack also get dex to attack (which, reminder, can only go up to +5), meanwhile, touch AC can bypass natural armor, armor, and shield bonuses, which many times exceed that +5 combined. For instance, My tank paladin has a 32 AC, but only a 12 Touch AC, and that happens with many enemies in the game, and that damage, since it's physical, is reduced, while magical damage isn't.

And, yes, there are enemies resistant or immune to fire, just as there are enemies (rogues and barbarians) which can sometimes not be sneak attacked outright. We are not talking about specific cases, we are talking about general base damage.