To answer the overarching question, I am not an electrician, but I think there’s a limit to how much power these devices can draw down at once from the mains, so they can struggle to charge multiple “heavy use” devices at once (plus usual caveats about cable quality, etc.). In the U.K. a domestic plug usually has an upper limit of a 13 amp fuse. Plug sockets themselves also have limits (cheaper sockets are less likely to be able to serve a maximum load). Finally, how your house is wired will limit the maximum draw available in each room per socket. (You are very unlikely to max this out charging a few Apple devices from one socket, I’m just explaining a bit of background!)
(By the way this is sensible even if it’s annoying, I don’t know about the U.S. but in the U.K. the second most common cause of house fires is electrics, and we have some of the strictest electricity standards of any nation.)
For example, I have recently bought a charging cube with 5 plug sockets, 3 USB-C slots and 2 standard USB slots. Each slot has its own max power limit, but the device itself also has an upper limit on how many “big” devices it can charge at once (it reduces power to all devices and charges more slowly if I plug multiple big things in at once), which is imposed by law and is a safety feature to prevent a fire. If I overload it, the power delivered to each device reduces, and I’ve noticed that with my iPhone and my Xbox controller, it essentially stops charging them until other devices are removed from the cube (which is probably sensible - it’s preferentially charging the more power hungry devices, which is probably what a user wants).
I touched on a caveat above but will just circle it again now - the cables we use matter! I couldn’t get a device to charge in my CalDigit dock recently and I was very irritated as it’s a new dock, but then I remembered people on here saying always check the cable before you blame the device and surprise, I swapped the cheap Amazon cable for the cable the device came with (it was longer and I didn’t need the length) and the device started charging! So it wasn’t the dock’s fault at all!