As far as I understand, the PC-98 architecture uses two video chips - one for text, one for graphics - and combines the output of both. This easily allows overlaying a static image with an independently running application in text mode.
On regular PCs, there is only a single screen and MS-DOS applications take ownership of the video hardware. Application-independent wallpapers aren't easily possible. Of course, Windows abstracts the video hardware and can do it - but MS-DOS does not.