The insanely huge movepools of many Gen 1 lines can be traced back to the very early development of RGB. As detailed
here, the initial set of Pokemon designed for the game weren't made with types in mind, as that mechanic was only implemented later into development. Hence you have examples like Clefable being able to use Flamethrower and Ice Beam, that make seemingly zero sense from a worldbuilding POV, but are too widely accepted to really cut at this point. Giving them stuff like Stealth Rock just kinda continued where the originals left off.