Can i write android tests in main source set? why not?

i know any android project has a package that i have to write my tests in it.

my question is why i can write my tests in main source set(package that app codes are in it)? for example create a class X that extend Junit and write unit tests in that class. or reverse , write app codes in other package? i don't try it but if you say i can't do it i want to know why?

maybe necessary codes that i need to do it isn't public or protected and isn't in other packages.

just i am curious.