It's not odd, it's the result of false marketing and frankly very much the direct fault of publishers. They spend an untold amount on promotional campaigns showing prerendered footage, gameplay running on workstations with unrealistic specs, and buy endorsements. All together that makes people invested and makes them expect too much. It's very easy to feel lied to and betrayed when the final result doesn't even closely live up to the promises. It's even worse for sequels, as they inevitably get compared to the original by its fans.

If the promotion wasn't false advertising there would be no backlash and people would act rationally, as you say. Nobody hates on something they've never seen before unless somebody tells them to expect more than they'll get.