As far as I can tell, because it allows one-man production teams. A single person can do everything, without needing to find a vocalist. It allows them to try m

As far as I can tell, because it allows one-man production teams. A single person can do everything, without needing to find a vocalist. It allows them to try more stuff quicker and easier, and anything that happened to get popular ends up covered by vocalists anyways (and the producer gets to sell those rights).