The main thing is that the monk's damage is dealt via consistent strikes of low-medium damage as opposed to single strikes of high damage. Every other martial class relies on GWM or SS to get their high damage, but increase their miss chance by 25% (except barbarians and rogues who can both reliably get advantage to offset this). So let's go on a math adventure.

Fighters and Paladins tend to use greatswords, barbs use greataxes, rogues use shortbows, and rangers use longbow.

2d6+3 gets us an average of 10 pts, GWM gets us an unweighted average of 20 pts, and a weighted average of 15pts.

1d12+5 (due to rage) gets us an average of 11.5, GWM gives us an unweighted average of 21.5, and a weighted average of 16.