Well that sucks. I mean as long as important warnings are also presented on the signs, why should it matter if someone slips in a silly joke now and then? Some people have no sense of humor.
The reason is that there are so many signs that
one can experience overload. I have at times.
Adding superfluous messages into the mix not only
exacerbates the overload problem, it could also
lead to paying less attention to such signs when
the actually have an important message.
It's analogous to designing airplanes for easy
yet competent cockpit management. Anything
extraneous poses a danger, & has resulted in
errors & crashes. Current design expertise has
come at great cost in lives.
It would be different if funny signs were billboards
set farther from the road. Drivers are accustomed
to easily ignoring those.