While answering another question, I stumbled upon a part of the script that reveals more information.
In S07E03, Sansa talks about the food stores of Winterfell:
(Sansa in italics)
How much do we have?
- 4,000 bushels, my lady.
What does that mean?
- For the current occupants of the castle, it's enough food for a year, perhaps more.
And what's the longest winter in the past hundred years?
- Uh, I'm not entirely certain. I-I'll check Maester Luwin's records. He kept a copy of every raven scroll.
You're telling me we don't have enough food, especially not if the armies of the North come back to defend Winterfell?
- No, my lady, most likely not.
This means that at least a significant amount of the forces has actually returned home.
There might be small exceptions to this, e.g. a subset of the Knight of the Vale who have stayed behind with Littlefinger, who seems to be residing in Winterfell for now.
So it would stand to reason that the forces have gone back home, and the Lords simply return whenever a meeting is held.
My original answer
Winter is here. There is no more time to lose. The Northmen are expecting an attack from the White Walkers at any time.
While this may be weeks or months, you need to observe how long it would take to reassemble the army if you let them disband for the time being. It would take weeks to get everyone back to Winterfell, fully armed and ready to go.
You need to distinguish between the lords and the armies here. An army is slow to mobilize, but a lord can travel more easily.
(edit the following paragraph is wrong, see update)
Instead, the army stays there, so that they can mobilize when needed. This also allows them to train. Since they are already feeling ill equipped to battle the Walkers' army, and they are essentially fighting for humanity, it stands to reason that they are training their men.
It has been mentioned a few times by Sansa that storing food for the future is a problem. But Winterfell is not besieged by any issues with its food production (at least, none are mentioned in the show). However, having a large army near Winterfell will make a sizeable dent in the food supplies, therefore depleting the food stores faster and requiring more food to be stored for winter.
However, about the lords staying in Winterfell, that could be different. It's possible that the lords return home inbetween meetings. There are considerable time gaps in the current season (Jon's travels seem to take no time, but he's traveling most of the continent. Compare that to Robert and Cersei's trip from King's Landing to Winterfell, which took weeks according to Cersei).
Then again, Littlefinger does appear to be staying there permanently. It's not impossible for some lords to stay in Winterfell, even if some others return home. This could correspond to how far they have to travel, or in Littlefinger's case, because he's gathering intel.