Convenience is 99% of the answer.
Plus, when "renting" a movie costs $3.99, and "buying" it costs $5.99, there's not a particular reason to not click the "purchase" button.
I wish more platforms would let you rent for $4, and then show you an "upgrade to buy for $3" or similar for a week afterwards.
Depends on your habits. I don't re-watch most of the movie I see.