i usually start my seedlings in soil, i didnt know the roots could be that much after only a week.
The answer's in your comment. While food, water and air are all important, the roots prime directive is to find anchorage. You gave it to them so, they stopped looking. Endo denied them, so they went searching. They wont stop until they have something to grab which, of course, they never will so they just keep growing and growing and... A lovely, vicious cycle.
I just started a new batch of seeds that are being maintained in Rockwool cubes until they sex themselves (keeping roots segregated in a shared DWC tub for 3-4 weeks is a royal pain) I was stunned at the slowness of growth. I'd forgotten just how explosive DWC growth can be.