Everytime someone does math like this, I want to cry at the time wasted.
Because it doesn't show anything in a white room vacuum. When a character has GWM, other party members will enhance that character because it is effective to do so.
So, if your math is "he's attacking the one the monk stunned or some spell restrained (advantage), is one of the Bless targets from the cleric (+1d4), is one of the two with twinned Haste from the sorcerer (another attack), if he misses he'll use Precision (+d to hit only if needed)", then you can compare we're seeing at the tables. Of course not this particular mix, but cascade of paty help (either buffs or debuffing foes) plus individual abilities like champion+advantage for 19% crit chance per attack which means a bonus action attack.
Also since it doesn't reduce the chance of crit at all (and gives a bonus on crit), it has great synergy with crit boosters (barbarian brutal critical, half-orc savage attacks, paladin divine smite, fighter champion). These can give even more reason for the party to throw buffs on the GWM character as more effective then a direct move.