Great question. If you can make a living off one album, sure go ahead. However album sales have dropped, which has put artists in a place where if you are indie, it makes sense to drop more rather than less. Its a simple formula called diversification. The more different products you have gives more people the opportunity to buy from a larger selection. Or if they are true fans, they buy more.
The thing is these guys have so much music just sitting around. Why not get cashed out for the stuff you do not think would be on your "target" mainstream album, but is still good stuff.
Hard not to sit back and let these guys do it. Shit, reggae artists have been doing that for YEARS!!!