I think, there's a lots of other stuff that makes some a big Hollywood star, apart from being a good actor.
Indeed, Arnold started his acting career because of his body, but to have this body (and be a champion of body building),
he had to invest a lot, and sacrifice a lot. He had to be disciplined and of strong will.
Probably, during the filming of his first movies he was equally responsible to his duties, and tried his best, and directors liked him.
Edward Furlong (John Connor in T2) mentioned how Arnie was very supportive to him, and patient,
and helped him a lot during the filming of T2, and where Furlong was just a kid. So, he is great colegue too
Add to that personal charisma I mentioned before, and no wonder people like Arnie so much.
Me included.