I totally agree that much of the time, it's just people wanting to work on something they find familiar, being incapable of maintaining code other than their own.

But then again, sometimes, the problem is just that the legacy code is shit. In a project last year, for instance, I had to rewrite every class I touched; the whole thing was so fragile that it was impossible to change anything without the rest breaking. So, I did this “incrementally” rather than all at once, and factored this into the cost of the features I was implementing for the client.