A lot of those single tracks you see up on Bandcamp are the musicians trying to raise enough money to finish production on work they play live.
It's much easier to play live than it is to record. Recording is a major financial stumbling block for a lot of new musicians, on top of the time and work that takes place inside the studio. Properly mixing a track requires a fine ear, advanced technical knowledge, usually takes at least as long to complete as it did to write and record the track. This gets expensive as well - there's a reason sound production engineers generally make more than musicians.
This is why signing with a label is such a big deal for most bands - having that money up front to complete an album, as well as the label's connections with recording studios and their engineers, removes the biggest obstacle they have for getting their music out there.