I look at that photo and see the roots of the plant exposed. That could cause the wilting you see. Are they all like that?

That plant is too old for it to be damping-off.

Sorry for the poor picture quality - those are just some pine needles that had fallen near the plant. The roots aren't exposed.

Could this be cabbage root fly? I don't want to yank the plants up just yet to see if there are maggots eating the roots.

Take another look. I would mound up soil around the plants, but this one doesn't look like a prize winner.

This looks similar to what pillbugs do to my squash plants. Look closely at the stem right close to the ground. If it looks like miniature beavers have been at work, you have your culprit.

And to those who don't believe pillbugs cause damage, save it. That's been exhausted on a couple of other threads.

Lots of things can cause wilting like that including over-watering, poorly draining soil, root maggots, harsh winds and temps, etc.

But what sticks out to me is how shallow planted it is. Even the cotyledons are way above the soil level. I'd either mound the soil up around it clear up to where the first true leaf joint is or carefully dig it up, examine the roots and if all appears ok then re-plant it deep like it should be.

Dave

I pulled the worst looking plant, and found it has almost no root system. I assume a plant this age should have more roots? I didn't see any evidence of pests.

For the future know that, If you pull up a plant, most of the evidence is still in the ground.
Dig it up is preferred so that you can carefully separate soil from root system.
That said, the dark color does indicate a root failure.
Now, the challenge is to figure out why.

Yeah, I just had a couple of cucumber seedlings to the same thing.