Although not everything listed in the MLS is listed on Realtor.com nor does it mean that it can be listed on other agents websites via IDX.
MLS systems provide data feeds for the active lisitngs to Members who can provide this information (via an IDX data feed) on their respective websites. It is not a data feed of all that exists in the MLS, but rather active listings. There are some exceptions when active lisitngs are not displayed. A broker can determine that they do not wish to participate in the data exchange.
Additionally, most MLSs feed the majority of their active listings to Realtor.com in some variation or form.