Why two seperataly doped semiconductors cannot be joined to form a junction?
At the interface of semiconductors, the silicon structure ends abruptly causing unwanted effect. Silicon atoms will not be able to make the usual 4 covalent bonds, causing electrons to be "trapped" more easily. These trapped states are annoying and will have several undesired effects. For all practical purposes, that destroys the diode characteristic. 

The whole idea of a PN junction depends on the flow of holes and electrons and how they balance out. However, in metals, there's only one game in town: electrons... Free electrons, and lots of it! Having a few irregularities at a metal contact is not that big of a deal if you have so many free charge carriers floating around. 
If you sandwich the metal between the junctions, however, you remove any "semiconductor interaction" as you force holes and electrons indiscriminately through that sea of electrons. Holes are immediately recombined, electrons get lost in the sea.

That being said, some rectifying behavior can still happen for a metal-semiconductor junction (as is the case for a schottky diode). But it will not be the same curve as a PN junction.