Political self-obsession and onanism
Even Boris Johnson has to be right occasionally.
The reason BT was privatised in the first place is that every time the telephone network needed major investment the Treasury had kittens about public borrowing and the plans were scaled back and shelved. There were waiting lists even to share lines; telephones and calls were expensive. And the government got the blame for it all.
BT is far from perfect, OpenReach needs to be split off and better regulated - and perhaps telecommunications service providers should get a stake in its ownership and management. But the idea that politicians can succeed with something they barely understand where everyone else has failed just tells you everything you need to know about politics.
Even if a Corbyn government were to make a major investment in broadband over a parliament, there is little likelihood that the necessary funds would continue to be forthcoming from future governments: the real problem with public ownership is that it's really hard to run a business when your priorities are reversed every few years. And future governments will get the blame for it all.
What we need from politicians is boring competence. Regrettably, that doesn't get much traction in an election campaign and it's the incompetence that's getting tedious.