True. I agree that many things PCs do while in the reach of a hostile creature SHOULD trigger OOA:Because it doesn't leave them more open to attack than other things characters do.
- Using the Healer feat
- Getting an item out of your backpack
- Attacking a second creature while engaged with the first
- And on and on and on...
With some spells, such as booming blade, without a somatic component and requiring an attack as part of the spell, really shouldn't provoke an OOA. Other spells, such as charm person, has a somatic component, but as players and DMs of the game we have no clue as to what that somatic component involves. It might be a simple waving of the hand, or it might be complex, involving gestures with other parts of the body, moving steps in a pattern, or whatever. Finally, some spells with somatic components have in their description the somatic action (such as burning hands).
But even then it depends on a lot of other factors. We don't have weapon speeds to compare to casting speeds, or how much does reach affect the situation?
Honestly, it is a difficult. And so most things could, or probably should, provoke OOAs don't. But it keeps the game simple, which was always one of the defining goals for 5E.