There are a lot of possible reasons:
- It's more secure
- The less freedom you have to change anything, the more you have to use google services
- Not allowing you to modify it keeps their brand "clean" and recognizable
- It makes it harder for you to evade any potential tracking/profiling and thereby allowing them to make money with your data
- They stay in control of the operating system, enabling them to force updates etc.
Google "walled garden" if you want to know more about all this. It's basically been Apple's approach for years.