One reason is in the documentation for the MEMSIZE option:
Note: Setting MEMSIZE to MAX is reasonable only if consumers of large amounts of memory are not likely to become active after SAS has started. For example, if multiple instances of SAS are running concurrently, and all of these instances are started with a MEMSIZE value of MAX, one or more of these instances can encounter out of memory conditions, or, the operating system can run out of available paging space.
You may not do it but it is very easy to have multiple SAS sessions run at the same time, especially if running BATCH programs. I have run as many as 25 batch programs concurrently (but not with such a large Memsize setting).
I have the luxury of not working with "large" data sets so have never actually run into a memory limitation. YMMV.