First of all, there's no such thing as a "Thunderbolt Display", because Thunderbolt isn't a display interface, but it carries DisplayPort signals along with PCIe and USB over multiple serial interfaces through a single cable. Sure there are displays that support a Thunderbolt interface, but internally they branch the DisplayPort signal into the display, and use the rest of the signals like any other Thunderbolt dock. They are essentially displays with built-in Thunderbolt docks.
The core of the question was why MacOS doesn't support DisplayPort MultiStream (MST), which is used to split DP streams into multiple displays over individual streams. Think of it like a network switch, but for a display. All display signals over DisplayPort are packet-based, which allow the signal to split off as needed to multiple monitors without loss of visual quality or introduction of compression artifacts, as long as the total bandwidth of the uplink can support the aggregate downstream display resolutions and refresh rates' bandwidth. The Thunderbolt "passthrough" that you mention is fundamentally similar to DisplayPort MST, just a different protocol and interface, but still dealing with packets. The downside of MST is it adds an imperceptible amount of latency (or so I read), but that's something that a Thunderbolt hub or "Thunderbolt Display" would also do.
Support for DisplayPort MST is not a hardware limitation with Macs, as running Windows on Bootcamp gives you MST support. It's just the lack of support in the OS, and again the core of the question here, why?
DisplayLink as was mentioned earlier is nothing like DisplayPort MST. One is fake, laggy and consumes host resources, and the other is natively supported by the DisplayPort standard.
Cheers.