Most likely in my mind because phone makers would like you to purchase a new phone every 2 years, and a new battery only begins to degrade around that time as well.
Eh you seem to like conspiracy theories. Not to say there aren't conspiracies out there, but for some people this is the first and only explanation for everything. As you can see in the other comments there are technical reasons for what the phone makers are doing.
I mean, I enjoy using Apple products, but they quite literally paid out a very large lawsuit which found them culpable for planned obsolescence with iPhone 5 batteries and etc.