They probably DO have the high res graphics, but making the game support them is too much work. The PC port is the PS game wrapped with a PlayStation emulation library and some tweaks such as 0-9 game speak and named saves. Since the PS does not support these high resolutions, they'd have to re-port the game to native PC graphic and sound libraries like OpenGL or DirectX. The English term for this is "Too much work". :P
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