What am I missing?

[Kent]

All non-residents had the choice to purchase a 2020 license as far back as Dec 1 2019  so if you choose not to until now, you have forfeited those previous months as a personal decision. It is no different for a resident if they choose not to purchase a 2020 license until now, they have also forfeited their right to hunt anything in the previous months of the year.
And even if you purchase a 2021 license on Dec. 1, it does not give you the right to hunt until January 1, 2021 and not Dec 2020.

I am the proud owner of a 2020 annual non-resident Idaho license, that I purchased several months ago.  Idaho is missing out (just like Utah did until the last few years) by not having their fishing licenses run from the date of purchase for the next 365 days, rather than expiring on December 31st.