[beambot]

Easy... money. Graduate classes are required to pull in top-tier researchers and grants, but they have relatively-fixed overhead costs for teaching (eg. professor salary). Want to maximize the return on that fixed overhead? Add more students. Many public and private universities are "Masters factories" for budget reasons. They typically filter (strongly!) between completion of MS and entry into PhD programs.

(Identifying such Universities will be left as an exercise to the reader.)