Did you water them properly, is the temp outside at a good point for germination...too hot..too cold.....7 days might be rushing the germination date...I don't really know the variety you planted but my variety says 5 to 14 days for germination...even if your packet says germination in 7 days...that's just an estimate.

My guess is that it dried out. Cabbage seeds are planted so close to the surface that the kind of heat we're having nationwide right now might be a little intense for cabbage. Generally cabbage is planted as a spring crop. Even if you did get it to germinate I'm wondering if it wouldn't bolt during summer heat.