Honestly, even though Larians approach is different from the 5e rules it makes more sense.. you don't decide to ''cast a spell'' at the mili-second that your blade touches an enemy or misses. When cleric casts guiding bolt it can also miss and waste a spell slot, why can't paladin miss their smite?
Balance reasons. Not all choices are necessarily made for immersion, but to make sure each character can perform up to par. Not that I'm saying all classes are perfectly balanced.
Rangers, whom are also half casters, have concentration spells that also don't fail just because their attack misses. Ensnaring strike lasts till you hit within a minute or your concentration is broken. If it works like you are suggesting, it would become a terrible spell, but it doesn't because of balance purposes.
Clerics are full casters and get access to many spellslots and cantrips. They can afford to burn a spellslot. It sucks if they miss, but they have the spellslots to spare if it doesn't work. Half casters can't afford it.
