Why would a grad school make admission offers prior to the deadline?
Sending offers too late to a very good applicant also involves the risk that the applicant will accept a different offer, and the university will lose out. Very good applicants also tend to receive many offers, so this risk should not be underestimated.

Similarly to the case of hiring employees at a company, if a university wants to admit the best students, the optimal strategy would probably be to make sure to admit the very best applicants as quickly as possible, while delaying the decision for the "good but not best" applicants until the end of the deadline, in case someone better shows up. The difference between study programs and hiring processes is that universities have plenty of statistics from previous years that let them know what a "great" applicant is and what a "good" applicant is, so the could optimize this process much more if they wanted.

In the German system, some universities have this explicitly built into their admission procedure. For example, at the Technical University of Munich, great applicants are immediately admitted, and good applicants are invited to an interview:


  In the initial stages, the grades you obtained during your Bachelor's program, as well as your written documents, will be evaluated using a point system. Depending on the number of points accumulated, applicants are either immediately admitted, rejected or invited to a 20 minute admissions interview carried out by the department faculty.