Because on the whole it takes time to actually impact a place. The amount of time varies by level. I'm not going to ding a junior for short stints, but if I see someone with 10 years of experience who never stayed at any company for longer than 2 years then I'm going to be extremely suspicious depending on kind of role I have. If I have a role that requires deep domain knowledge and relationships to be effective, pass. If I just need someone to come in and crank something out, then I'll check them out to see if there's any notable red flags.