The energy suppliers wouldn't want the support burden of having to troubleshoot people's Wi-Fi, which might not reach to the location where the meter is installed, and could suffer from all sorts of problems.
Some consumers wouldn't like the idea of it using their internet connection - even if it was a negligible amount of data consumption.
I believe there are some energy supplier provided wireless networks that cover a small part of a neighborhood and all of the meters in that area can connect to the network.
I think there are also mesh models where meters communicate over a local network wirelessly, back to a meter that is in the cellular coverage area.
But yes, mesh networking might be an interesting alternative.
I wondered about radio; just have these things constantly emit their signals, and then slap a radio receiver on a cell phone tower to collect them. Or maybe some other frequency; I don't know that radio is ideal. There are some privacy concerns too, if people don't want that info to be public. Could always be encrypted.