Why are there duplicate names for streets in cities?

I recently began participating in a massive user-created question-and-answer site called Quora.  It was designed by former Facebook CTO Adam D'Angelo and has a similar feel to the social networking site.  I picked some topics I was interested in and they show up on a feed similar to the wall.

The other day one of the users asked

Who ensures that cities don't duplicate street names?

There were already two answers that claimed that no one in city government approves street names, because there are lots of duplicate street names in cities like New York.

Well, actually there is an arm of city government that approves street names, the department that approves plats and development does this.  However, I do agree that there are lots of duplicate street names and the reasons why are somewhat complicated. 

The problem is that some streets are developed on a piece meal basis.  In San Antonio if a developer wants to open up a piece of land for development, in order to have access to the land he/she is also required to build a road to connect it to an existing street.  The city has a Major Thoroughfare Plan that maps out where the city wants major streets to extend, so if a developer is building a road that matches the plan it will be called by the name on the plan.  But the developer is only building a part of this road and it may not extend all the way to the existing roadway with the same name.  Eventually the road will connect, but not until the property in between has been developed. 

Another problem is older cities, like New York and San Antonio were built before city planning was invented.  Cities just grew organically with no rhyme or reason.  In San Antonio many downtown streets were former cattle trails and are very disorganized.  In order to change a cattle trail into a street with a grid pattern the streets have to jog at right angles, thus causing a street to end at one point, jog a few streets over and then start up again.

Street names also get duplicated when an incorporated area is surrounded by the city limits of a larger city.  The smaller incorporated town has its own jurisdiction and platting approval mechanism. The larger city may have some influence over street names but sometimes relationships are contentious.  The other possibility is that the smaller town existed long before the larger city surrounded it and the street names were already in place.

An addendum to the question popped up a day later asking

Why don't cities correct duplication of street names?

It is very expensive to rename an existing street.  Not only is there a cost to the city for signage, plat changes and legal fees, it affects a lot of private businesses and citizens as well.  Anything with the current address has to be changed, maps have to be changed, all contacts who are aware of the old address have to be advised of the change, etc.  And then of course, there are the streets that exist in incorporated towns surrounded by the larger city that can do as they please.