Because shareholders and valuations. If you don't expand expand grow expand grow grow, people immediately see this as the company collapsing this very moment. It's impossible to just "exist". Be stable. Be reliable. Nope, massive growth every year no matter the cost or you're obviously doomed.

Because too many online companies have their value predicated on continued growth and so once they saturate their niche they try to continue to grow and doom themselves in the process outside of a select few that had already become too big to fail.