They don’t have room for another building the size of the data center to store batteries in and thousands of acres of land for solar panels (wild ass guesses on battery building footprint and solar field footprint)

Critical loads require generators, batteries don’t cut it. Data centers want the most reliable backup power they can with the longest runtime. Battery storage density is not high enough to back up a 500MW+ data center for any length of time without a comical amount of batteries.

When the NEC allows critical, equipment, and life safety branch at hospitals to be backed up with batteries and solar panels, battery storage will be at a point where battery backup of data centers is feasible. Right now it isn’t.