Because if they slow down people won't upgrade as fast because the manufacturers pledge "X number of versions" updates not years. So if Google slows down manufacturers would have to support longer. Gotta keep that money flowing in because Google gets a cut of sales.
I never claimed they did, but the fact remains that my iPhone 4S was rendered nearly unusable after the iOS 7 "upgrade," which had such a massive system load that only one app could run at a time.
They are hurting themselves in the long run. People get bored of continuous updates and it means that each release gets way less attention. They should do minor version bumps if anything. Do a 14.1, 14.2 and so on