Why do streets and highways have speed limits rather than velocity limits?

They actually DO have velocity limits. There are legal restrictions on both speed and direction.

- - Speeds are limited according to the black numbers on white signs that you see on sign-posts everywhere.

- - Directions are limited by the layout of the pavement and curbs on all the highways, avenues, roads, boulevards and streets, as well as the countless signs that say "One Way", "No Left Turn", "Keep Right", "Keep Left", etc. Violate one of these, and you get nailed as sure as if you had exceeded a posted speed limit.