Main reason I ask for detailed bios is threefold. First, some ideas people have for their character either don't work for the lore or the setting, or they are otherwise problematic without revision. Second, it helps me understand their inclination to take OOC information into account to make a suitable character for the premise. You can tell when somebody wasn't reading if they're including details that contradict what was outlined or flat out ignores something vital. Third, it helps gauge writing ability. When you are taking on 6-8 characters and there's over a dozen individual applications, it's the smallest details that might give one character a pass over another. I know from personal experience that I almost always get an absurd amount of applications and I even write critiques for each of them, but I cannot as a GM manage too many players without being extremely superficial and unfocused. 

Point is, long bios absolutely have a place and aren't pointless. If a game dies prematurely, it's very likely a number of other factors that contributed.