You get into some funny (and seemingly unfair) rules once tax exemption is involved. It's why you have limited rollover of unused FSA funds in the US--and the ability to rollover at all is relatively recent.
And yet HSA funds are the most tax advantaged of all. No tax going in, can be invested in anything that can be bought via Fidelity, and no tax coming out, if used for healthcare expenses at any point in your life, even for your dependents.
