I use EndeavourOS too. The main reason Endeavour isn’t considered Arch is due to tech support reasons, mainly. Even though it’s using Arch’s repositories, it’s more appropriate to get support from the Endeavour community/developers rather than through the distro it’s based on.

Anyone who says otherwise is just being an elitist. I picked Endeavour since it’s a quicker install process. I’ve installed Arch enough times from distrohopping now. :p