Because it actually validates your address. Plenty of “official mail that you receive at your home address” can be accessed (or produced) without access to the listed address, but you can't spoof knowing information that you never received.

It would be reasonably easy to check my mail before I do every day.

Or maybe I conspire with someone that doesn't live here but wants to appear to live here (which can obviously also be done with utility bills).

I'm all for making it as easy as possible to vote, I'm analyzing the properties of the piece of mail that the government sends.

If you want to make it as easy as possible to vote, you can't have a permanent address requirement as such. As has been said, many, many people don't have one.