After mulling on it for a bit, I think I might’ve figured out why fujoshis predominately ship the two dominant male characters in shounen manga. For starters, all too often, the chemistry between the main character and the intended love interest is way too shallow to be interesting, especially compared to the effort that goes towards showcasing the bonds between the two male friends/rivals. As a result, shippers gravitate towards the more interesting relationship, regardless of whether it was intended to be romantic or not. As for why they go to actual bl/yaoi manga, if the romances in those manga are often written like the sub par romances of straight characters in other manga, then it probably means the characters only like each other because the other is cool or they have a passionate gaze that lights a fire in their heart or whatever excuse used to pair these two characters together instead of writing an actual relationship with more complexity and depth than a puddle on a sunny day.
At least people are wizening up these days, and romantic writing shortcuts just aren’t gonna cut it anymore. So actual good romances are being written more often.