Some do sure, but not all of them! I watch an equal amount of both male and female streamers across all games and sometimes I prefer hearing a female's voice and approach to a game over a males.
To name a few female streamers that I watch and while granted they do have some nice cleavage they don't openly flaunt it: {at least I don't think they are}
DizzyKitten
Yuuie
Mssnowhite
AnneMunition
LegendaryLea
Lynxaria
Aurandra