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It seems to all revolve around the TCP/IP stack; X was written to be network-transparent, so all X programs have to be, too. Plus, the original Amiga system software folks had the luxury of being hardware-dependent, which the designers of UNIX and X could not afford to indulge in.
I'm not making excuses for X and/or UNIX; but I suspect that if the Amiga's designers had had the same goals for the OS and Workbench that the designers of UNIX, X and GNOME/KDE have (that would including designing Workbench so that there was no default look-and-feel), they would have come up with much more bloated software than they did. But they couldn't; in fact they were so impressed with the design of the Amiga that, having been contracted to write system software for what essentially was a simple computer-cum-console, they came up with possibly the most advanced 1980s OS *outside* of UNIX/X.