If you mean the actual underside of the chip where the pins are on the actual processor where they touch down into the socket, it's likely because they are rather fragile and will bend/break easily. Always hold a CPU by the edges.

Bending or breaking a CPU pin can render the entire chip useless.

i meant the gold connections on pci cards

The same principle applies.

Skin oils can act as an electrical barrier, and are also acidic and can make the brass connections wear to a point where they aren't properly conductive. I do a lot of PC work, and do commonly touch areas of the system that is technically a no-touch zone, but with a bit of preparedness you can make sure you do no harm. A soft pencil eraser is excellent at removing all sorts of residue, and an alcohol wipe will clear out any left-over oils.

Most contact points are now made out of a high-copper brass alloy. Gold is usually reserved for high-contact areas within the micro-electronics these days.

thank you