The FARs are clear that different medical rules apply to glider
pilots. A glider pilot indeed does not need to hold any medical
certificate, and the FARs do not require that he has not failed a
medical certificate. He is only required to be able to honestly
believe he is fit to fly on the specific day he wants to fly.
I'm not sure why the FAA made these rules. My guess is, as an
extreme example, an illness that will cause death in one or two
years would mean that a 3rd class FAA medical can't be obtained,
even though the pilot may still be perfectly ok to fly that day, and
for many months to come. The FARs are written to allow a glider
pilot to use that time to legally fly, on all days when he honestly
believes he is fit to fly. Like all pilots, holding a medical certificate
or not, glider pilots are expected to ground themselves on any
days when they are not fit to fly.