Baglady’s got it right, regarding “bankability.” Ben Affleck is hot, so Hollywood execs try to put him in everything. Gary Sinise is a dependable professional with good references, and he’s got an impressive resume, so he gets a lot of work. Jennifer Love Hewitt, on the other hand, can’t really “open” a movie, but she’s got experience, so she’ll do a smaller number of lower-budget films.
There’s another factor to consider: An actor can do many more movies in a year than just about any other film professional, except perhaps producers or writers. A director will do one movie every two years, or eighteen months if they’re lucky. A cinematographer may do two; an editor may do two or three (though they might do piecemeal work on several others).
An actor, on the other hand, works on a movie for anything from two weeks to two months, depending on how involved their role and how complicated the film might be. For example, Sean Connery’s bit-part cameo at the end of Robin Hood: Prince of Thieves (an extreme example) took all of two days. Faye Dunaway’s small role in the Thomas Crown Affair remake took maybe a week. On the other end of the scale, Ed Harris worked for four months on The Abyss, and Keanu Reeves was involved with The Matrix going on six months (including kung-fu training).
On average, though, an actor can discharge his duties on a film much more quickly than anyone else, and move on to the next one. Some actors, Harrison Ford notably, carefully choose their properties. He has lots of input in the production at all phases, so he’s involved from early on, unlike, say, Harvey Keitel, who considers himself strictly an actor, and works on several projects a year. Ford also looks at his alternatives from a marketing standpoint; he wants to keep his appearances limited, so as to avoid saturating the market, and increase demand. (Of course, he also hates the business part of the industry; he’d much rather hang out in Jackson Hole with his kids.)
So consider this equation: An actor will appear in more movies each year if two factors coincide: (1) They’re extremely bankable, and (2) they like to work a lot. Put those together, and that’s what you get.