Labour under Corbyn has already lost two elections, including getting humiliating 13.7% in the European Parliament poll. One of the reasons for such an extremely poor result was Corbyn's confusing position on Brexit. It has changed a bit now, but it's still much more unclear than the positions of the Tories and the LibDems.
It seems that the great majority of Labour MPs are determined to stop no-deal and/or hard Brexit. To do it effectively, they need a leader that can make the party win the election. Why is Labour sticking with the leader who arguably cannot win?
In the current situation, a possible election would become a vote on Brexit. Corbyn is not trustworthy for a large number remainers, who make up a large percentage of Labour's electorate. Moreover, there are a lot of voters who could consider voting Labour, but will never accept Corbyn as a prime minister. Why won't Labour choose someone who would be acceptable as a leader of a cross-party anti-hard-Brexit coalition?
This Guardian opinion piece goes into more details on the above arguments. However, I still haven't found any answer to the question posed in the title: Why is Labour sticking with Jeremy Corbyn?
@mikado says that Corbyn has support from party membership. But why does he have support? Labour members are overwhelmingly pro-remain, and I can't understand why a remainer would support Corbyn, who 1) is a Brexiteer himself 2) loses election after election and makes hard-Brexiteer Tories more likely to win.
@Dan Scally: "You're seeing things through too much of a brexit veil"
Yes, I understand that a lot of people support Corbyn because of policies not related to Brexit. But right now Brexit is the single most influential event for the future of the UK, and when it's done, it will be something that nearly impossible to change for a long time. Is nobody in Labour thinking strategically? I mean something like "let's choose some centrist for now, so that we can easily join forces with the LibDems and others, and when we're done with the problem of Brexit we can choose Corbyn or another left-winger".