Ah yes....a very perplexing question. How does one not forget how to ride a bike. WELL, I believe I can find the solution.
First off is the mount. In order to mount the bike you must have incredible amounts of dexterity to lift you leg over the seat. Luckily for us humans we use our legs everyday, and because the weights of our egos are rather hefty our legs have developed into quick, powerful machines. It would be impossible for us to lose this first, and very important, ability.
Second is the grip. In order to ride the bike you must keep a firm grip on the handlebars. Again this is fairly routine for most people. Males seem to be especially good at this part, and thusly are usually better bike riders. This is another fairly easy step.
Thirdly and most importantly is the peddling. Now, for some people this is rather tricky. Those who are klutzy and often forget how to walk right usually have SOME trouble remembering this step. I'm determined however that due to our genetic structure, that has been passed down through generations back to the cave men is where we find our answer. You see, it is basic human instinct to run from danger, as it is with most animals. And we humans weren't always on the top of the food chain. Since we used to be frequently chased around by the dinos that inhabited the earth we have deeply ingrained in our minds the ability to run. And luckily for us running is almost identical to peddling.
And there you have it....riding a bike is part of our genetic structure at the most basic level. This makes riding a bike impossible to forget.