(Most) Fast Pokemon are designed to be Sweepers or Baton Passers and need speed to overpower opponents. The 50 fastest Pokemon (albeit legendairies) have Defense and Special Defense stats below 100.
Take Aerodactyl for example. Aerodactyl has plenty of powerful moves in its level-up movepool, including the Elemental Fangs, Bite/Crunch, Iron Head, Rock Slide and Sky Drop. Its TM pool is even more vast, encompassing Dragon Claw, Earthquake, Giga Impact (with Rock Head), Stone Edge and Fly.
With such a vast variety of Physical moves and 105 attack (135 with Mega Evolution), Aerodactyl would be an unstoppable beast with defensive stats of Blastiodon (168 Def, 138 SpDef). Game Freak realized this and gave it defensive stats of 65 Def and 75 SpDef to counterbalance its amazing Attack, making it less of a threat.