Benedict Cumberbatch who played dr strange isn't really pretty neither was adam west playing 60s batman
Cumberbatch may not be classically good looking, but he has a unique look, unique features. Different from other Hollywood actors, plus he is a good actor, hence his fame and the demand.
To break into Hollywood main man roles, or main supporting roles, you have to have at least one of 3 things basically.
1)Be extremely good-looking, and not just regular good looks. The kind of looks where a random person stops and stares because you are that good-looking
2)Have some unique things that you bring to the table. Unquestioned charisma, screen presence, etc that can't be taught
3)Undeniable acting abilities. Someone like Daniel Day-Lewis caliber, where you go to see his acting skills, not his looks
The reality is people don't want to see less than good-looking faces on screens. People want to see handsome faces for men and beautiful faces for women. It is pleasurable to see them on screen.
Face it, you'd also admit that you would rather see a good-looking face on screen than an average or ugly face that you see everyday on the streets.
Good-looks and sex appeals sell all the time, and since Hollywood is like any other business entities, driven by economics. Of course main actors/actresses and main supporting roles go to good-looking people.
It's also a brutal industry, as there is so much competition for such few roles. Chris Hemsworth and Henry Cavill (both Gigachads by anyone's definition) struggled to break into Hollywood. Hemsworth dealth with anxiety for years before he landed the role of Thor because the job was so unstable. Cavill worked in bars and minimum wage jobs before he broke through as well.
There are often literally over a thousand people auditioning for even a tiny supporting role in a middle--of-the road budget movie, and there is only one to go around.
