Because the chances of Prime Minister Corbyn in 2020 are approximately zero. An election campaign fought by a 71 year old man whose politics have been repeatedly rejected at the ballot box by the UK electorate, with five years of age shift meaning that the electoral influence of the baby boomers, who are mostly fairly comfortably off, will be at its absolute peak.
A focused, united Tory Party, led by a 49 year old George Osborne (most likely), versus a grey-haired pensioner who gets rattled by being chucked soft ones on C4 News? C'est magnifique, mais ce n'est pas la guerre: c'est de la folie.
Corbyn is wildly popular on twitter, amongst people of whom there aren't many and they don't vote anyway. Examples of campaigns that were wildly popular on twitter: AV and Scottish Independence. Remind me how those votes turned out?
The best Corbyn could do would be to stack up larger majorities in what are already safe Labour seats, plus (possibly) winning a few Lab/Tory marginals. That's not enough to form a government, assuming (as is likely) that the SNP don't implode in fratricidal killings in the next five years.
I have some sympathy with Corbyn's policies in the abstract, I voted Labour in 1983 so I'm one of the people that a similar set of policies didn't alienate and I don't necessarily see policies of the left as inherently impossible. But the reality of an Osborne/Corbyn general election in 2020 will be five more years of Tory misrule, and if you really care about the vulnerable, rather than wanting Labour as a talking shop, a compromised Labour government that can do some good is infinitely preferable to a pure Labour opposition that can do nothing.