First, those unique MACs and serial numbers are not currently in storage that requires an electron microsocope to read, so that's a pretty big additional cost burden. Second, assuming all devices were to be given secure key storage parts, you also have the cost burden of the pairing process during manufacturing and maintenance, as I mentioned above (not to mention the design and development of that pairing database and its failure/diag/maintenance/factory-reset modes). It's far from trivial.
If you don't provide a convenient interface to read a MAC address then you're going to need an electron microscope to pull it off a NIC chip as well. They just always provide the convenient interface to get at it.
No, you don't need an electron microscope to overcome that type of inconvenience, because those pieces of data are not sensitive and no effort has been made to ensure you can't just read them out using the pins. This is why the problem of storing private keys is so different than the problem of storing a MAC address. Or put another way: inconvenience is not security, and what we're talking about is a security problem.
