Because weeds steal plant nutrients that you are feeding to the grass and compete for the root and growing space needed especially for the creeping grasses. Some weeds while being mown back simply adapt to growing more spread out than up and compete for more room in that manner. Weeds seed out every year and the seeds are just waiting for their chance to germinate and the product must be in the ground before the full development of the plant for the best success before you see the weeds.
Weeds are just regular plants who are out of place.
What is a weed for one person or crop area, is considered a valuable plant to another person. For instance, Johnsongrass is considered a hard to kill weed, but some farms actually plant it for its foraging value. Lawn Weeds - Controlling with the Mower. Over time as your lawn grows and spreads out - the grass itself can choke out the weeds. You can assist with this by keeping your lawn area mowed regular and at the correct mowing height.