It's true that the old school left Corbyn comes from are anti-EU but the fact is that Brexit is going to happen under the Tories and if the Brexit true believers win their internal battle it's going to be shaped in a fashion that trashes employment rights and market regulation. The hard left want to sweep away the EU for exactly the opposite reason, to live out some kind of retro-Soviet-lite.
The European market has been an economic success while incorporating social protection and market regulation. You could make a decent case that it lacks transparency and accountability in its power structure, but the result has turned out pretty well. It's also made it much more difficult for UK governments to make big left or right swings like they used to.