Why don't black boxes send a signal when above water?
The short answer is that when a plane crashes on land we (generally) have a pretty good idea where to look for the black boxes - there will usually be a relatively clearly defined debris field to search.

When a plane goes down in the water there's a lot more ocean to search, and currents can deposit parts of the airframe over a much wider area, so the underwater locator beacon ("pinger") helps guide people to the debris field.



The longer answer is that sending a signal when you are underwater is relatively easy.  The pinger has a water switch: when it gets wet the water closes an electric circuit and the pinger makes a noise that can be heard on standard passive SONAR gear.
Water is pretty good at conducting sound waves, so the pings can be heard over a large distance and can guide searchers to the black box (or at least the pinger: sometimes they get ripped off, but they're usually still near the debris field).
We get all of that functionality in a fairly small self-contained device that requires effectively no maintenance (you replace the unit when its battery expires).

Sending a signal when you're NOT under water is harder: you can activate the signaling device with a "G Switch" the same way the ELT is activated, but after that the problems get harder:


You can't use sound (effectively)
A "pinger" beeping in the air is much less effective than under water - the range would be extremely limited even at very high volumes. Add to that the fact that crash scenes are pretty noisy with firefighting and rescue crews working and the chances of anyone hearing an acoustic beacon on the surface are pretty slim. 
You can't use light (effectively)
A strobe on the black box may work OK in the dark, but it's easily damaged and not terribly effective during the day, or if the recorder is upside-down, in dense foliage, under snow/mud, etc. 
Radio locator beacons are problematic
Radio locator beacons require an antenna - if the antenna is damaged or torn off in the crash the effective range of a radio beacon would be drastically reduced, possibly to the point of being completely ineffective.
If the antenna remains attached the signal can still be reduced drastically if the box is buried under natural features (dirt) or aircraft debris (aluminum skin).
(Both of these issues are frequently raised in criticism of ELT technology.)




It's possible a locator system could be devised that works on the surface, but given the challenges and the fact that the location of the black box is generally known (to within the size of the debris field at least) when an aircraft crashes on the surface there hasn't been much effort invested in this.  

Finally, remember that while the recorders are a very valuable part of the investigation process remember that from an investigation standpoint it doesn't matter if you find the black boxes on the first day or the last day of recovery operations:  Finding the recorders will not undo the crash or bring the dead back to life, so if it takes a little longer to find them while sorting through a debris field on the surface it's really not a major problem.