The land beneath a building does not deprecate, whereas the value of the building should.
Since for most properties the majority of the value is in the land itself, your basis to deprecate would be much lower, and more realistic.
1. The land value.
2. The structure value.
3. The right to have this structure on the lot.
That is, if you tore down the structure you wouldn't generally be able to build anything anywhere near as large, because the building predates zoning.
Assessment ignores (3), and groups it in with (2), which then depreciates. Which isn't right, because (3) doesn't depreciate
I'm not sure that's the case for the majority of the country. At-least it's not here in my greater metro area (suburbs or rural... yuppie downtown areas... perhaps, but again, downtown is not majority for most places)
Under usual market conditions, houses are headed to being on the older side and reaching the end of their effective lifetime, leaving little value left in the structure. You can renovate a home to bring it back to new-like condition, which restores value to the structure, but that cost must be maintained in the equation.