Because someone on the team that wrote it thought that you don't need that capability.
Install MyDirections. It makes adding, finding and editing locations a lot easier, and when you select a location it runs Maps set to go to that location. (Go to settings/preferences in MyDirections and set Direction type to Driveway (if you're driving, or to whatever other transportation mode you're using) and Default action to Navigate.)
It's a free app, and it makes Maps about as usable as any GPS on the market (and better than a few).