There’s all kinds of trade-offs you’re ignoring/assuming away.
Cool, name a convincing one. So far, you've failed at that.
Please bear in mind I'm talking about video games here - some of these limitations you have in mind are somewhat more valid in a PnP environment.
Spell slots aren’t resources? The most scarce resource there is is character slots and if you’re burning one on a buffer you’re giving up a threat.
That might be an argument if you couldn't rest at-will (or practically at will by returning to town, etc.), making spell slots effectively unlimited, which is the case in almost all games.
Hell, if you are not a sorcerer (or favored soul-like class) you even have every spell at your disposal after nothing but a long rest to change a slot. Encounters that actually burn through your entire spellbook all at once are... exceedingly rare.
But even if it wasn't so and the game actually made rests rare (more games should do that, btw, for many reasons)...
Unless you are doing a very specific build, generic allround buffs are almost always your best choice, because they are almost always more useful than more situational alternatives.
Having a bless-like pre-combat buff as a cleric or a similarly generic pre-combat buff as a mage is
never a bad choice.
Guess that is more or less my problem with these pre-combat buffs like bless - beyond the annoying "should do the pre-battle buff ritual....".
They are too generic, too all-round. Even a fireball can be useless against the right (or, well, wrong) enemies. It's not that rare to have element-resistant enemies.
What's the "counter" to a bless? An enemy that slings non-stop buff-removals? Has that ever happened in a game?