macOS does support MST! It actually does! But it only supports it for splitting one display image over multiple streams, to overcome bandwidth limits on Displayport streams.
For providing a signal to very-high-pixel-count displays, macOS uses MST.
For providing multiple displayport signals to multiple displays? Nope, not implemented.
Imagine my frustration after a day of googling and finding out that macOS supports the feature we wanted, but not the use that we wanted.
You're mostly wrong; MST does not increase bandwidth, it splits the bandwidth of a single DisplayPort link. To increase raw bandwidth for 5k/6k they combine multiple HBR2/HBR3 links (over Thunderbolt or with multiple cables), which is the opposite of MST.
MST is supported specifically for early 4k displays that had scalers that couldn't handle 4k60, but could handle half the resolution, so they sent two streams. But that wasn't a bandwidth limitation; old MST 4k displays and modern SST 4k displays both used a single HBR2 link.
That is basically what happens when you don't support MST.
The screens light up from the same DP stream, which is also why you can have, say, 2x 4k@60hz monitors in this configuration where true MST would need to drop it to at least 30Hz, or lower res.
macOS does support MST! It actually does! But it only supports it for splitting one display image over multiple streams, to overcome bandwidth limits on Displayport streams.
For providing a signal to very-high-pixel-count displays, macOS uses MST.
For providing multiple displayport signals to multiple displays? Nope, not implemented.
Imagine my frustration after a day of googling and finding out that macOS supports the feature we wanted, but not the use that we wanted.