Why doesn't AMD (or any of the testing sites) include the maximum video resolution of GPUs?
Because not even the most powerful GPU right now, the 2080 Ti, can game at 4k60 across the board, so there is very little reason to go beyond that.
If all you want is to know the maximum resolution, you only need to look at the websites of the manufacturers of the cards. The Sapphire RX 5700 XT, for example, tops out at 4096x2160 on HDMI, and 5120x2880 on DisplayPort.
It's also listed in the specs on the etailer website, as well as on the product boxes. Generally if it's a standard resolution, supported by the HDMI or DisplayPort revision, it's supported by the card. Pre Navi cards had issues with very oddball ultra compressed widescreen resolutions, but otherwise they were only limited by the HDMI and DisplayPort revision limitations.
For example:
Newegg:
B&H