Because there isn’t any real market value because you are not constantly selling your house. Also does the tax estimate diverge that much from reasonable price?

Because it’s all made up bullshit.

Probably because the assessed value is what you are taxed on, so it is usually lower and more conservative of an estimate because people with houses don’t like taxes (and maybe have more political will than renters). You can google this stuff, what did you find? I found this article: https://www.forbes.com/advisor/mortgages/assessed-value-vs-market-value/