Knowing that a time limit is on all Microsoft exams, why is that I can
prep for an MOUS exam online but not be able to have an actual REAL MOUS exam
I'm not 100% sure, but I believe the issue lies with Microsoft's delegation of the MOUS program and its exams to a third party. Because that third party has not elected to deliver its exams through VUE and/or Prometric Testing, as is the case with MCP exams, they've made other arrangements with iQcenters to deliver the exams in a paper-based form.
Practice exams can be built in any arbitrary form (as is also true for MCP exams and many other cert exams as well), so that explains why you can take some of them online.
This was first published in February 2002