The driver doesn't necessarily notice the special note every time. Sometimes the driver might not think the doorbell is a big deal or may not understand the meaning behind that note. One way to interpret this is "Huh, doorbell must not work, and maybe the customer is upstairs or in the basement, or in a place they otherwise won't be able to hear a knock." Sometimes if you really jam your fingers in a broken doorbell it'll actually work. So many the driver tries the doorbell just in case before he decided to knock on the off chance you're able to hear the knock.

Once upon a time I had a note that said to just leave the pizza on the porch. Sometimes customers order delivery and then suddenly have to leave the house for some reason, so they call back and say to leave it on the porch if it's already paid and tipped. I assume the customer left the house in this case. Well, it was a busy night, took a while to get there, maybe they're back home now. So, I knock / ring the bell, and whaddaya know, the customer is home! Except she's pissed because she was on a phone interview and didn't want to be interrupted. ._.

At the end of the day, you don't have to tip. It's not mandatory. But it is proper etiquette, it's expected, and if you're a shitty tipper or don't tip at all, the drivers will remember you. They will ensure you get shitty service. They'll get back at you. They're not all like this, but many of them are. If you tip poorly or not at all, it's in a driver's best interest to encourage you to take your business elsewhere. The store's bottom line is irrelevant, only our tips matter. This is, of course, just one side of the coin, and every pizza place is different. Some places are serious shit shows where employees are able to get away with a lot and take extreme liberties. Other places are more up-tight.