The answer to 'lawn fertilization has weeds' is that weeds can still grow in a fertilized lawn for several reasons. Applying fertilizer alone does not prevent weeds from germinating and spreading. Weeds may still take hold if there are bare or thin spots in the lawn, the soil pH is imbalanced, or the lawn is not mowed at the proper height.
To get rid of weeds in a fertilized lawn, you'll need to take a comprehensive approach that includes overseeding, adjusting the soil pH, and using selective herbicides or hand-pulling weeds. It's often best to call a professional landscaper who can diagnose the specific weed issues and create a tailored treatment plan for your lawn.