Agreed it's weird to normal people but given the context that those who criticise early leavers are actually praising themselves "ooooh look at me i stay right to the end - #Superfan" it's a perfectly understandable viewpoint.
They're often needy self obsessed types out to prove that they're a "better" fan than you whatever that means. Taking this early leavers theory a stage further to involve leaving after the final whistle is a natural progression. Then it'll be after the last player leaves the pitch / last staff member leaves the pitch / lastaway fan leaves their section etc etc