Am I missing something, or do these devices not exist?
If so, is there a reason they don't?
They certainly are not popular. That's because there are few use cases for them. The Apple iDevices with a Lightning port are not built to handle more than one device at a time. The Belkin Rockstar only allows a specific pairing of devices, one that provides power input and another that has audio output. It is in effect emulating a specific kind of peripheral to the iDevice, an audio sink and power source which is a common style of an iPod dock.
I'm curious why the USB-C-to-USB-A adapter that I have (with the ordinary USB-A-to-Lightning cable plugged into it) is incapable of charging the iPhone.
That sounds like a hardware issue to me.
Perhaps you are merely concerned that the time it takes to charge is not what you hoped. Apple computers source only 15 watts from their USB-C ports, and depending on the cable and other factors the computer may source far less power. That could appear like it's not charging for someone used to a 30 watt or larger charger.
Anyway, I think my second bullet point still stands: I can't find any hubs that offer Lightning ports (at least one charge port) and have a male USB-C cable to plug into the MacBook.
This is a very narrow use case. Any Apple computer should be able to provide 12 watts of power to the phone while plugged in with a USB-C cable, it's not 15 watts because every USB-C cable I've seen will use the old USB 2.0 protocol for data and power negotiation with a computer. The people that will want or need more power than that are few. The iDevice models that can sink more than 15 watts of power is also few. Creating a Lightning hub (or whatever such a thing might be called) would be complicated as it would have to "know" which port it is supposed to draw power from. Adding logic to gauge which port can source more power would be complicated, and labelling one port "data only" and the other "power only" is likely to confuse and frustrate people.
There are Lightning "hubs" much like you describe but only to provide a specific function, that being for sourcing power and sinking audio. The market for a "hub" that does more than this is small and the complexity, and therefore cost, of them likely high.