No, it is because Bill Adkinson, who also wrote QuickDraw, was a programmer, not an artist. More to the point, there is no desktop in MacPaint. Nor are there any movable windows. It takes over the entire screen and OS. This was made before multitasking arrived on the Mac.
To the original question, the MacPaint format had a major limitation other than the 1-bit bit depth. The file dimensions are a static 612 x 792 (8.5" x 11" @72 DPI). Actually, it may have been a little bit larger. Those numbers do not seem correct.