To be able to sell their devices in countries not supported by Google Play (China).
As a leverage to get better licensing deal from google. Manufacturers must pay Google to include Play Store in their device, and it can cost up to $40 per device. By having their own app store, phone manufacturers might have some leverage in licensing cost negotiation (e.g. Google offering them money to not include other app store).
That’s clearly not what I was asking. The question was rhetorical because they DON’T prevent other OEMs from installing their own app stores.