I think writers have a tendency to do that because they write what they know. Sure, they can dream up characters (quirks, characteristics, etc.) by eventually, especially if these are main characters, they're going to be drawn from people the writers know, or at least readily supplied by his or her psyche. After all, characters -- as real as they are -- are only extensions of the author's imagination.
Especially if you have written 15, 20 novels, how can you ever avoid writing a similar character? After all, I don't think there are really that many fundamentally different types of people in real life anyway. Sure, we're all different with different backgrounds, etc. but in many ways, a lot of us are very similar in hopes, dreams, temperaments, personalities, etc.
In my own writing, I try different things, and the characters who speak to me seem to be very different. The protagonist in my first book is very different than that in my WIP, but they still share similar traits (partially because they're both the "heroes"). My female leads are rather different, partially because one is still very young (15).
Personally I don't make a conscientious effort of making my characters all different. I don't draw character charts and say, "Hey, I already have someone like this, so I should make him this way so that he's different..." I just let my characters speak to me and show me what they're like. I understand there would be limitation, but at least they feel organic to me instead of some puppets or quirky characters for quirkiness' sake.