Mac Address Lookup Pointless?

Short and sweet. Relatively new CCNA and it is bugging the crap out of me that there does not appear to be any detailed information as to why some mac-addresses do not exist on oui-ieee.org. I have seen some talk of there being an unmanaged range of mac-addresses that devices can self generate for certain purposes but

1. Does this not defeat the point in having a unique identifier if it can be spoofed?

2. Where is this information listed? How do I determine which is and which isn't a spoofed mac-address and is there anyway to trace a vendor from these pointless addresses or should I just ignore them?

I ask as I have port security violations constantly being flagged and when I go to trace the device, it turns out it does not exist.