You would think that an opposition party that is in disarray, is confused over its leadership, and is way down in the polls, would rather wait until the planned date for the next election so that they can get their ducks in a row.
That would require Corbyn to say "We can't possibly win a general election under my leadership, so are declining the chance to put our case to the electorate". Not something a party leader wants to say.
(Maybe there'll be a backbench rebellion of labour MP's)