Unfortunately, from what I've been tol, RCT and its followups are heavily, heavily dependent on Windows and Intel-specific code, so much so that a Macintosh version would basically have to be rewritten from scratch. That makes such a project expensive and time-consuming enough as to not make it worthwhile for the parties involved.
but we do have Sim Theme Park (but it is only good for about 8 hours of entertainment and is finished quickly)