It's quite simple: They have to make a Spidey movie every X number of years to keep ownership of their existing license. Ones with Spidey himself are a guaranteed bank, but because they're tied to the MCU now, they have to play ball with Marvel Studios on scheduling(but the profit outweighs the annoyance I assume). If they have to make a Spidey movie to stay current, but can't use the man himself, then this is pretty much their only option. The losses due to a bad movie can be written off for tax breaks, and may very well be less than the profit they make from the MCU Spidey movies that they get a piece of due to keeping the license going.