it's because the pins either on the chip or under the chip are very fragile and easily bent/broken. once this happens a lot of the time you have just trashed either the chip or the mobo depending on the brand.
no force is needed anyway since the chips just falls into place and locks in with the included lock mechanism.
same with the heatsink, enough force to hold it in place is good, too much force and you can break the socket or mess up the pins, or both.