Why are people unable to develop lasting immunity against Norovirus?
So the direct mechanisms of norovirus immunity aren't well understood - the shortage of straightforward animal models, and the extreme difficulty in culturing the virus make mechanistic studies really rather hard.

A modeling study pegged the duration of immunity at 4.1 to 8.7 years when talking about symptomatic disease, which is much higher than many of the estimate's you've encountered, and isn't actually all that fast. The older estimates were built off human challenge studies, which involved massive doses of norovirus, rather than the normal exposure you'd have. Now that is for a single strain, but immune cross-reactivity for strains isn't well understood. There's also several efforts to develop norovirus vaccines, which suggests some duration of immunity is at least possible. It's also possible that a proportion of the population (~20%) is simply immune due to the absence of the FUT2 gene, which acts as a receptor for the virus in the intestines.

In addition, there's definitely some antigenic drift, where mutations in the primary antigens negate previous immunity to the disease (the same mechanism that causes you to eventually lose immunity to a particular strain of influenza).

This paper is a pretty brief review of all these factors, written by some norovirus experts.