It's because they almost always have something
silly on them.  Since they're shown so small on
the TV screen, and held for such a short period of
time, the animators figure they can just put any
darn thing on the acetate, and nobody'll notice. 
Unless... they own the cel.  ;)  My Gundam
Sandrock blueprints, for example, have lines of
BASIC game code in the text boxes.

But this one, I think, might just take the cake. 
*giggling madly*

Here's a zoomed-in view of part of my latest
acquisition, a blueprint of Byalant from Gundam Z.
 As you can see, it has such important components
as "vacation," "again," "yamato," and "magical
penguin special."