If GWF is the end-all-be-all must-have, how is it possible there can be groups playing 5e--without the feat--managing to do just fine?
Because not every group has the same goals in playing the game, nor every player, and different DMs run different encounters even when running identical modules. Some groups don't mind at all that the Fighter, Barbarian, or Paladin runs away with damage in melee combat, or that the Ranger and Warlock run away with ranged damage. That's perfectly fine to them because it's not what the game is about to them, or they're happy that those classes get to dominate combat (i.e., they enjoy the imbalance).
Or, if you'd rather: the same reason in 3.5e people played anything other than full caster + prestige classes; the same reason in [classic] 4e that people played classes other than Fighter, Ranger, Warlord, and Wizard and didn't take tax feats.
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For my group, we don't like -5/+10 as a mechanic. It makes people at our table feel like they're not contributing to combat. Also, it makes people feel like they have to stop and do algebra in combat. It makes our tables less fun for the players and the DMs. However, we really enjoy feats because they give martial classes something to do with an ASI after level ~8. So, we changed the feats:
GWM: Eliminate the -5/+10 ability. Replace it with: "When you make a melee attack with a Heavy weapon and hit a creature of at least size Large or larger, you deal an additional 1d6 damage." I'm a fan of 1e/2e two handed swords dealing 3d6 damage. We haven't played with this enough to know if it should be limited to Huge and larger yet.
Sharpshooter: Eliminate the -5/+10 ability. Replace it with: "Your ranged weapon attacks have a new range category: Extreme range. The limit of extreme range is equal to double the weapon's long range. You have disadvantage on attack rolls made at extreme range." The range is not that much different than Spell Sniper. We haven't had a problem with cover in our games, so we haven't modified that at all, but I'm not opposed to changing it so 3/4 cover is instead 1/2 cover. We'd probably change Spell Sniper, too, if we did that.
There are other feat designs that we're not really fans of, but we haven't changed them because they haven't caused enough problems. Polearm Master's bonus attack, the lack of spear entirely, and the random inclusion of quarterstaff. Crossbow Expert's credibility-straining "ignore reload" ability. The fact that Resilient lacks a "you make take this feat multiple times" clause. The fact that Heavy Armor Master/Heavily Armored and Medium Armor Master/Moderately Armored aren't each 1 feat that says, "You gain ~this~ armor proficiency. If you are already proficient in ~this~ armor, instead gain +1 Str or +1 Dex to a max of 20." The fact that Lightly Armored exists. The fact that Durable and Tough aren't 1 feat, etc.