I haven't even looked at much of anything vaccine related much less have any expertise, but the explanation I remember hearing at some point is that by priming the immune system our bodies can prevent the virus from getting established. Polio and smallpox are also hard for the body to fight off but were some of the earliest vaccines (smallpox being the first according to Wikipedia). I don't think it implies that it is possible to create vaccines in cases where our body can fight the disease but then does not retain that resistance over time, although I would guess there is at lest some chance that is possible.

A better way to look at it, for me, is to start with what a vaccine is doing: it's not giving you resistance to a pathogen.

It's triggering your immune system to produce and/or store things, the presence of which then confers resistance when subsequently exposed to the actual pathogen.

So from that perspective, the next question is, "Do naturally occurring pathogens, presented in a natural manner, produce an optimal triggering themselves?" To which the answer, as you might guess from the messiness of real-world biology, is "No."

It turns out that while amazing our immune systems are also general purpose and adaptable, and such systems can get confused, or go off on a tangent, or focus on the wrong thing, etc etc.

Vaccines interact with this system by balancing two things: (1) how do I trigger the strongest, purest immune response that's effective on the thing I want to immunize against & (2) how do I avoid triggering such a strong immune response that I convince the body to attack itself?

It turns out, with modern biology, we actually can do better than nature on (1). Even if (2) still requires some guesswork (and a lot of testing).

I wouldn't assume that the reason resistance doesn't last in some case is just due to suboptimal triggering, although as I said I don't know much of anything about vaccine development. I do often encounter people with inflated opinions of modern medical understanding that does not match reality, so I am suspicious that your claims are not necessarily grounded in reality. I'm also not interested enough in the topic to look into it in any detail. I do agree that it isn't necessarily impossible for a vaccine to cause lasting resistance even if direct exposure does not.