What has this got to do with elephants (or goliaths) being naturally stronger than mice (or halflings)? That the difference is NOT merely cultural! That a mouse (or halfling) raised by elephants (or goliaths) will not be stronger (on average) than an elephant (or goliath) raised by mice (or halflings)?
OK, but that has nothing to do with allowing people to assign their +2/+1 as they wish.
1: There's a max cap on attributes in 5e
anyway, which means that even if all goliaths started out with +2 in Strength and no halflings did, they could still both end up with 20 Str anyway. Just not at the same time.
2: Strength, the ability score, is not a direct comparison to strength, the effect caused by muscles. It's an abstract ability to determine how much extra damage they do with weapons. Also, Small creatures can't use heavy weapons without a hefty penalty, which means they're limited in the amount of damage they can do anyway.
2a: In the real world, many small creatures are actually proportionately stronger than larger ones. I've read that a mouse can lift up to twice its body weight and can easily support its weight with one paw, while an elephant can't. Mice can also jump and climb--both functions of Strength in D&D--while elephants can't.
3: Goliaths (and firbolgs, bugbears, orcs, loxodon, and centaurs) are always going to be naturally stronger than halflings (and gnomes, goblins, and kobolds) because goliaths can lift and carry things like Large creatures and Small creatures get a penalty to lifting and carrying things. So even if a halfling has a higher Strength than a goliath, it still won't be able to out-lift a goliath. And most people in the real world consider lifting capacity to be a better indication of innate physical strength than the ability to hit people, which is seen as a learned skill.
3a: Lots of tables barely even care about encumbrance or lifting abilities anyway, except at those in-game times when they have to lift a gate or bend a bar. And that's an Athletics roll, which is a skill that all goliaths have.
4: These rules apply
only to PCs, of which there are usually no more than 4-6 in any given world. Assuming that any of those PCs actually are halflings (when there are
so many races to choose from), then having one halfling be a muscle-bound steroid user among the tens of thousands, hundreds of thousands, or even millions of halflings in that world isn't going to hurt anything--nor will that one 198-pound goliath weakling.
5: Assuming a player even
wants to be a a super-strong halfling, of course. Like the game-breaking influx of mountain dwarf wizards that never happened, there aren't likely to be that many players who desperately want to play a super-strong halfling. But there are going to be a few, so is it really that big a deal to let them?
6:. A bucolic halfling rarely has to carry a lot of heavy things at all, besides the occasional keg of ale or particularly large wheel of cheese. A halfling raised in a more strength-based society
would develop a more muscular frame than one who wasn't. Likewise, a goliath raised in a culture that didn't require a lot of physical activity would be much more physically weak than one raised in a "traditional" goliath culture.