It's because on a brief period of old PC hardware (1994-1998), when Linux was starting off, high-resolution video initialization (SVGA+) was easier to do in 16-bit mode, before the bootloader switched the CPU over to 32-bit mode. Because GRUB handled this transition, GRUB became responsible for video initialization. After 1998 GPU manufacturers added 32-bit support for initialization, but the long tail of users with hardware with that limitation really entrenched this responsibility in GRUB.