Huge 2-stroke Diesel engines designed for ships would be much more efficient for electric generators than jet engines, but they also would need much more space.

Nonetheless, I doubt that the xAI datacenter had such footprint constraints that it really needed the smaller jet engines. Perhaps the time-to-delivery for really big Diesel generators was too long, so they opted for the already available repurposed jet engines.

The efficiency of the generators with jet engines could be increased a lot if the heat from the exhaust gases would be recovered and used to generate extra electric power, with a steam turbine or with another kind of turbine with Rankine cycle, e.g. with organic vapor.