It isn't that Apple doesn't understand. They completely understand and make choices to do A better rather than A+B so-so. I wish more companies would follow suit rather. It's strong product design (the overlap between product management and design in its true sense).
There may come a time where a competitor has all the features of FaceTime (especially its ease of use) and have platform agnosticism. Then the decision might change.
If the likelihood of a person using platform A is P(A), then the likelihood of two of them communicating is of the order of P(A)^2. So even if you have 20% platform penetration, that only gives you a communication likelihood of 4%, leaving plenty of space for other providers to fill.
Airdrop is only useful IF:
1. You want to send a file to someone
2. They are standing right next to you
3. They have a fairly recent Mac, running Lion.
Whereas Dropbox is useful whatever OS they're running, and wherever in the world they are.
Perhaps they are attempting to create a lock-in via network effects similar to Microsoft's where everybody feels they should own at least 1 apple device just to be able to be compatible with everyone else.
It's also possible they will port this to other platforms once they have it right on the mac as there are many people (such as myself) who simply can't justify the cost of buying an apple device.