This was a great feature of the Macintosh II in 1987. Sad that it's not ubiquitous yet. (Even modern Macs snap apps to one monitor or the other, so no video walls.)
Linux, and I believe Windows as well, can display windows across different monitors just fine. But sometimes the monitors are too different, and part of the window gets ugly. Probably a blocker in Appleverse.
The vast majority is just bloat. Unimaginable bloat, hundreds of levels deep.
I can play video games at 4k 10bit HDR 120Hz with a 1kHz mouse polling rate. I can copy data at 10Gbps across computers via 30m of copper. I can transfer at 1.3Gbps through wifi.
But it takes 250ms for the f'n calculator to start up on a good day?
This is a general issue to all modern software.
And you can certainly have such features in modern / fast frameworks. Take a Java Swing or JavaFX app and compile it with GraalVM Native Image. The framework dates from the 90s but is maintained because JetBrains relies on it. It supports hi-dpi displays, accessibility apps, multi-monitor rendering and so on. And when natively compiled, it will start in the blink of an eye.
Like, there shouldn’t be anything fundamental that prevents it from working (it just has platform specific native dyn libs which works fine with graal), but never seen an example, so I would appreciate a pointer.
https://github.com/gluonhq/substrate
Sibling gave a link for Swing. I've also seen it done for Jetpack Compose but that's definitely more experimental.
None of this stuff is very smoothed over though. There aren't many people doing it. But, the results are pretty magical.