The thing is, with today's technology, Steven does NOT have to be in the same country where post-production takes place. All he has to do is go into any decent recording studio, have a digital version of the movie running so he can dub, say his lines, and be out of there in half a day, all done.
The real reason is that he simply can't be bothered, especially for these low-budget pieces of rubbish he's inflicting on us. He stepped up for Into the Sun because it was originally supposed to be a theatrical release, and therefore it had a decent budget (and probably a producer who had a firm enough hand to make him do it).
Considering that even for these low-budget flicks he's making he's still taking in a cool 3-million dollars per picture, I should think for that fee, he could spend half a day in a recording studio re-recording his dialogue.