Why does I watch a movie doesn't make sense while I watch movies makes sense?
While I go to church makes sense and I go to churches doesn't.
The singular "I watch a movie" refers to an individual, single, discrete action.
What do you do to relax after a hard day at work?
--I watch a movie. idiomatic
The plural "I watch movies" refers to a habitual or regular behavior.
What is your worst time-wasting vice?
-- I watch a movie. unidiomatic
-- I watch movies. idiomatic
go can combine with prepositional phrases with a singular object to express the idea of habitual or regular behavior in a specific role related to the object of the preposition:
I go to school.
I go to church.
I go to choir practice.
In these cases, the roles are "to attend as a student", "to attend as a member of the congregation", "to attend as a member of the choir".
When go is combined with a prepositional phrase with plural object, the meaning is different. go then means to make your way to these places as a visitor:
In your job as building inspector for the Department of Education, what do you do?
--I go to schools, and make sure they are safe and that their systems are functioning properly.
When you visit a foreign city as a tourist, what do you see first?
--I go to churches because I like architecture that tries to express something spiritual.
As a talent scout for the big music label, what do you do?
--I go to choir practices in the hope of discovering the next big star.