Not sure why it is not a game toggle, since the option to get rid of crosshairs while aiming is in the XML file.
If you don't have a problem with editing XML files, you can find the option in "Steam> steamapps> common> 7 Days To Die> Data> Config> Items". Make sure you make a back up file first, it's not my fault if you break your game. Once in the file, search for "crosshaironaim", and change the "true" to "false". I leave the one for the bow on, and turn the others off. Just my personal preference.
After that you will want to go to "Steam> steamapps> common> 7 Days To Die> Data> Config> item_modifier", search for "crosshaironaim", and do the same thing. The item_modifier XML is where the laser sight and scopes are. If you don't change those as well, then you will have crosshairs again as soon as you attach one of the mods and aim.
You'll have to redo the settings on updates. There is/was a way to not have to redo them, but it's been long enough that I've forgotten what it was. But it only takes a few minutes anyway.