Long-time Thinkpad users scoff at that ... while the figure of a duck suddenly enters their minds..
(Yes: I clearly remember my pre-USB thinkpad having a graphical BIOS with windows, icons, and a duck-shaped mouse cursor).
BIOSes of the time were all written in highly-optimised Asm, and I suspect those little "easter eggs" they added were because the programmers knew they had enough space left over to put some more fun stuff in.
There was also AMI WinBIOS that provided a GUI, but I remember it being much less featureful than other BIOSes of the time with a TUI and didn't like mobos that used it, so in that case they may have sacrificed functionality for appearance.