I had not noticed this before, but apparently the question has already been asked of the GRO as per this URL:
https://www.whatdotheyknow.com/request/gro_indexes
It would appear that GRO imposed a 100 year limit on the availability of births when they first constructed the searchable index. This seems to have persisted in spite of there being post-1983 births now available on the GRO site and sites like FreeBMD making available post-1917 births. Apparently there was also meant to be a 50 year limit for deaths.