Because the seed starting mix has only enough nutrients to last it through the very early sprout stage, not long enough for the plant to reach the leafing out stage. Kind of like sprouting a seed in a paper towel... it works but it doesn't feed the sprout.

Seedlings don't need food until they develop true leaves. Until then food is supplied by the cotyledon. Then give only a small amount of weak fert. I usually use ditute fish fert, if anything.

Karen