If I were to venture a hypothesis, it is because riding a bike really requires "rewiring" inside the brain, as an all-body activity and involving a large amount of muscles and coordinating them. Which also is why it takes a while, unless you already trained to get the required balance control through some other means (the Asian martial arts fantasy movies come to mind, people running over easily rolling tree trunks floating in water - if you can do that you probably need less time to pick up riding a bicycle).

Knots on the other hand probably mostly only use the existing wiring scheme, so unless you have really bad hand/finger coordination you don't need to change how the muscles (of only arm - slightly, hand, fingers) are activated and used.

Purely intellectual knowledge is easily forgotten unless used, but activities that required certain neurons for proprioception and muscle control to find new connections remain much more easily - also takes a lot longer to gain.

Knots are easy to forget unless you actually understand the knot.

For example, I can tie a bowline as well as most bowline-like knots (ex: sheet bend) without a recipe and even come up with variants on the fly, and I don't think I will ever forget that. This is, of course, after tying a lot of bowlines and forgetting recipes dozens of times.

I think that's for your "riding a bike" analogy. Really understanding a knot takes time and effort, but allows you to remember and even improvise. Memorizing a recipe is faster, more efficient, but easier to forget.