Shuttle flew on autopilot. The crew was largely there to supervise, at least in terms of the flying up and back down to landing. Automated systems actually flew the craft. The pilot was the emergency backup plan. So Buran wasn't all that much of a leap.

Shuttle's crew did the things the autopilot could not, all the docking, all the satellite grabbing, and certainly all the science. Buran's one flight was just an up-and-back proof of concept. So a crew wasn't needed.

Without a crew, the pilots' only job would have been to save themselves in various emergency scenarios should the systems fail. Take the pilots out and you don't have to train/study/prepare for those scenarios, at least not before the first flight. By removing the pilots, Russia reduced early program costs. It was a form of technical debt. They would eventually have to prep for humans, but at least they could get the craft flying first.