Maybe they figure those types would be too overpowered with more HP? I mean look at Ferrothorn, it's got the typical great Defense as seen with Steel-types, but also pretty good HP -- and it's really good.
Ghosts I think are meant to be more as glass cannons in general. They have low HP but usually have high Speed and/or Special Attack. They can live long using shenanigans like Substitute, but they're really not meant to be walls (except Jellicent, which kinda sucks anyway, imo).