I have close to 2000 landings in retracts and I dont remember ever coming even close to gear up ( I am very forgetful BTW ) I attribute this to the initial training during my first complex checkout and the instructor drilled into me to lower the gear to slow down on downwind. So first thing I do when I start to slow down is to lower gear. I do this as second nature even if I am busy with other things.
Given that you are forgetful, how can you be certain that you were close to having a gear-up landing (or even did land with the gear up), and just don't remember it?
On the original topic of how you forget to lower the gear, it's really easy once you get through the first step, which is assuming that it can't happen to you.
I was thinking about this thread while doing a bunch of landings in my Arrow last week, because there is a feature of the plane that encourages you to tune out the gear horn. In my Arrow and probably others, if you have two notches of flaps and the gear up, the horn blares at you. The short and soft field takeoff procedures both call for two notches of flaps. If you dump flaps before raising the gear, you don't climb very well over the trees. So every short or soft field takeoff involves the gear horn blaring at you between gear retraction and flap retraction. It only takes one landing where you think that sound is normal for you to scrape the belly.
If you think it's far-fetched for people to tune out loud noises like that, spend a night in a house with a different surrounding than you're used to. Someone who lives next to the elevated railway will have trouble sleeping in a country house with owls hooting and crickets chirping, while the owner of the country house would have trouble sleeping next to the tracks. Neither of them is even aware of the noises when he sleeps at his own house, and the other will adjust within a couple of nights.
Now, add some distractions, a feature of most gear-up landings. The distraction can be as small as looking for where the avgas pumps are while you're on downwind or as big as holding a full barf bag between your legs, which was filled by the crying kid in the back seat while you have to go around when a spray plane takes off directly toward you when you're on short final with a failed alternator and a stiff breeze from the door not being fully latched.
If you haven't watched this video, I recommend it regardless of your feelings about Flight Chops: