To brag means to talk. To talk means to expend energy
Being humble often means to shut up. To shut up means to not expend energy => easier and more cost effective
Why would I do something if I could instead do nothing

We have this old folktale around here. There's this guy who sees his neighbor digging a ditch with a pickaxe, but he's doing it while sitting in a chair. So he goes and asks "Why are you sitting down while digging?", to which the other guy responds "Cause I tried to lay down and dig, but it didn't work"
Point is, be humble folks. Don't waste your efforts running your mouths around people. Nobody cares anyways.