It's useless and divided but 40% of the electorate will support the Tories as long as they say they are delivering Brexit. It's not an ordinary mid-parliament because the country is split down the middle on an incredibly contentious issue. Yvette Cooper or any other Labour leader imaginable wouldn't be able to convince Tory voters to switch no matter what they came up with. It's also difficult to imagine that the Lib Dems/Greens could be squeezed any further or gains made on the SNP.
This is fundamentally correct: you've got an issue that's incredibly divisive and which most people . When you factor in that fundamentally (ignoring divisions in both parties and stuff that people don't really care about outside the bubble) nothing has yet really changed; we don't know what's going to happen with Brexit and similar issues, and so the polls have not shifted. Add in the fact that you've seen polarisation behind two parties and that there isn't really an alternative force like the Lib Dems or UKIP for people to go to anymore and it should be expected that the two parties remain relatively close together. The last election really shows how polling can shift through a campaign though!