Parliamentary elections are more complicated than that, had Labour won three or four most seats they would have likely been in power as a coalition with other parties of the left. This might have quite literally cost Labour the election.
Reality is Labour lost the election despite going up against an incredibly weak opposition and basically refusing to discuss or take a position on Brexit, which was an irresponsible form of electioneering. When faced with a competent opposition they took the same approach and were wiped out. That's on Corbyn.