This is a little unusual. Most games never explicitly say you need an SSD or a HDD - but Starfield does! This likely isn’t a hard limit, as recommendations are often just that, but I cannot help but wonder what would happen if the game is run on an HDD?
Not having an SSD is a major bottleneck in performance for any system built in the last 7-10 years already. It’s really not an unreasonable requirement.