Male-to-male Lightning cables & Lightning hubs
Am I missing something, or do these devices not exist?

They exist...
https://www.apple.com/shop/product/HKKP2ZM/A/belkin-lightning-audio-charge-rockstar

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 &quot;know&quot; 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 &quot;data only&quot; and the other &quot;power only&quot; is likely to confuse and frustrate people.
There are Lightning &quot;hubs&quot; much like you describe but only to provide a specific function, that being for sourcing power and sinking audio.  The market for a &quot;hub&quot; that does more than this is small and the complexity, and therefore cost, of them likely high.