There is no logical reason...people always seem to think it doesn't apply to them just everyone else.
In this situation, I would print a reminder that says something like this "On the reverse side of this page is a copy of our daily schedule. Since everyone has previously received a copy, this is just a reminder for your convenience. Please note our meal and snack times, especially breakfast. Breakfast here at daycare is from 7:00 - 7:30, afterwhich I clean up and move to the play area. Food is not allowed in the play area so those arriving at or after 7:30 should finish eating prior to be left in my care. Also, while I realize that things come up at work and/or traffic may cause a delay in your pick up time I do request that you notify me immediately when you realize you will be late rather than right at pick up time. These things should be the exception and not the rule. Thank you for your time and cooperation". Be sure to put the schedule on the reverse side. Print one for each person and an extra to have them ALL sign. Then if it continues, you have to give the violators a direct warning..."Liz, since Johnny hasn't had breakfast you will need to take him back upstairs and sit with him while he eats and clean up when he's done. Our mealtime is done." If it happens again.."Liz, you need to take Johnny home until he's done w/ breakfast". At that point, you may need to reconsider keeping Johnny in your care.
Please keep in mind that people sometimes run late. If their kid is used to eating at your home and they get up/arrive late, they probably didn't have time to add eating at home to their morning. I can definately see myself taking breakfast to the center...my daughter doesn't eat when she first gets up and getting her up earlier really isn't an option. I would seriously consider moving your breakfast time to a bit later...maybe you don't serve it after 7:30 but allow until 7:45 for kids just arriving to finish what they have brought. That is what my daycare did. She didn't serve breakfast at all but as the kids arrived, they would sit and eat if they hadn't already or finish up but they were all done by 7:45.
Another thought is if the late pick up is occassional, it happens. If it is a chronic issue, you should include it in the reminder (as a showed above). Then often repeats should definately be charged a late fee if you don't already. My old daycare had late fees but if you called her and said "I'm running about 5 minutes late leaving work" she didn't mind on occasion (I had exactly enough time to walk out the door right at 5 and get to her within a few minutes of closing). If the phone rang just before 5, I couldn't get out the door right at 5. She knew I was coming straight there.