Why do I have to respec just to keep the spells I already selected before.
Because you miss the part where trading a spell is OPTIONAL.
You don't have to do it if you don't want to.
Again if BG3 would simply allow trading the newly selected spell, this problem could be completely avoided.
But that's not how the mechanic should work. You don't know the spell you're picking on level up, thus you can't trade it.
