For places like Middlesborough or Burnley they're a shadow of their former 80s selves. Austerity there is just a case of rubbing salt into the still open wound.
Those jobs aren't coming back, whether we are in the EU or not.
But you have to consider recent history to understand life for people there. It's not just "stupid bloody Northerners voting for Brexit" because they don't understand the world has moved on.
Many once prosperous, vibrant, manufacturing towns were decimated under Thatcher. The councils didn't have two brass Farthings to rub together (both from the loss of business and also ever increasing centralism, especially of funding). The regional redevelopment agencies (as I think they were called) managed to create a few call centre jobs and poorly draft some folks' CVs. Heseltine wanted properly funded development agencies, similar to the old New Town corporations, with enough power to make decisions and achieve things, Howe (I think) didn't want to spend the brass. If memory serves, Heseltine was able to make some significant difference in Liverpool (mainly because of the recent Toxteth riots) and Manchester. He wanted to cover all the affected towns but wasn't permitted both from a financial point of view, and it was interventionist which went against the religion of Thatcherism/Reaganism.
Then came large, poorly integrated, immigrant communities, actively placed there by one government or another (I forget if it was mostly Tory or Labour). There was tension, mainly stemming from the pre-existing lack of opportunity. Little wonder there were race riots in some of these towns a decade later.
Then came New Labour, full of promises. No regional redevelopment, so sorely needed for well over a decade. More London-centric than even the Tories which defied belief. More large infrastructure projects, but none within 100 miles. If London wanted the tube rails gold plated you'd believe it was more likely to be approved than development spending in Sheffield or Truro.
Children of the eighties grew up with no working parent as role-model. Opportunities heavily curtailed from lack of money, be that nutritionally, books in the house, or ability to go to university. It's little wonder many became disaffected and disenfranchised.
The three bed family semi might sell for £80k or £120k, if it sells, but want to relocate to MK or London or anywhere with some life chances? Can't even afford a shared shed in such places, let alone relocate a family. Hell, the differentials got so large they can't even afford to relocat to Manchester or Leeds
So along come the blatant lies of Johnson and Farage: "Take back control of your lives", "£350m a week on the NHS", "xx,000 immigrants a year taking your jobs". It was an easy sell after 30 years of bull in the media about stupid EU regulations, straight bananas, EU super state and what have you.
As another commenter insightfully pointed out receiving EU redevelopment funds is a reliable indicator of neglect by successive governments. That those places most heavily voted leave is ironic, but a reliable indicator of how neglected they feel by all in power for the last 40 years.
There's no point pointing out they were backing not just the wrong horse, but the wrong race. Tell that to Farage and co selling a vision they neither cared about or had any intention of delivering. Look at their surprise the morning after when they won. Not a damn lying one of them even smiled. About time the ASA covered political messages too.
Brexit was change. And maybe, just maybe it would be a change for good. Staying and project fear did not offer hope.