Is "dudebro" even about masculinity though? It's making fun of frat boys, which is a more specific thing, with connotations of immaturity. I don't think an equally masculine term like "sirgentleman" would catch on as much without those associations.

It may have begun with frat boys, but it’s clearly metastasised; if you replace all occurrences of dudebro with frat boy the results are incoherent.