They don't own the OS. One of the most powerful forces in consumer software is the ability to set defaults. The vast majority of users will never change the default browser, launcher, or app store; most don't even know it's possible. Google's position of controlling Android allows it to be wildly more corrupt and incompetent than any competing app store and still remain the dominant one. If another store does manage to gain a strong foothold, there are any number of OS changes Google can make to hamstring and stifle the competition. The situation is extremely asymmetric.