My impression is that he likely has more than enough money to retire, and so isn't fussed about joining a Championship team or relocating for the sake of it. Holding out for a move that interests him, either financially, or location wise. Similiar for De Gea (although obviously De Gea would have better clubs interested, but also De Gea is likely considerably wealthier as well). Maybe after a few more months they will get restless and just want to play, but i also wouldnt be surprised if they just retired.
I'm surprised we don't see it more frequently honestly. Players who have played at the top have, if they have been even semi sensible, multi-generational wealth by the time they're 30 or even mid 20s. De Gea might have, at a speculative but not unreasonable guess, somewhere in the region of £10m in the bank. Possibly considerably more. He could easily accrue £500k+ a year or so passive income from interest/dividends/investments etc for doing nothing. Lingard I doubt has that much, but enough to live off comfortably. And that's without considering they likely have financial advisors who have invested money on their behalf wisely, or having assets like property that have accumulated in wealth. I imagine there's also 100s of footballers who have turned millions into multiple millions through property or investments or crypto even.
I suspect over the next few years/decades we will see more players retire early. Some will obviously be motivated to play at a lower level for the love of the game (or because its expected/its all the know), but others with outside interests, or who don't want to relocate, I think we'll see more and more retire earlier.