This decision was made to minimize the bandwidth required for a given level of performance. Coordinates usually are declared as an int in the interface. Values larger than 16 bits are truncated silently.
Keyboards are the greatest variable between different manufacturers' workstations.
If you want your program to be portable, you should be particularly conservative here. Many display systems have limited amounts of off-screen memory. If you can, you should minimize use of pixmaps and backing store. The user should have control of his screen real estate. Therefore, you should write your applications to react to window management rather than presume control of the entire screen.