Because their routes are optimized based on all packages. They will have to deal with customers who start questioning why the driver didn't take xyz path which would have delivered his package faster (Though may result in slow delivery for everyone else).

Perfect transparency is great in an ideal world. But we (Humans) are not all that matured :)

I guess the company owners would argue against it based on security concerns, especially if the courier is delivery packages that have high net worth

If it's security concerns you can see UPS and FedEx trucks every day on the road, just follow one - or even just watch them leave the hub!

So that logic seems incorrect.

But you don't know what is in the trucks.

djhworld is talking about a scenario where someone at a company knows a certain high-price shipment is coming in. He could talk to accomplices and direct them to the exact truck and intercept/steal the item before it's delivered. Who would know there was a connection between the theives and the insiders?

On a more practical note, I can understand why UPS/FedEx wouldn't want consumers to see this. They would be bugging the hell out of the customer support reps asking why their truck wasn't stopping at their place first, etc. Sure, if the driver is lost, that's another story, but I think the larger companies already track their equipment in realtime. The signature pads are always live now.

They're trying to deter the lazy criminals?

I think you can currently do this by using google latitude, all these trucks will need is a device capable of checking into google latitude. A raspberry pie with WIFI should do, and for protocol to send back data they can use MQTT. Also read about the idea of internet of things, might interest you.