Because the purpose of copyrights and patents is to enchance society (i.e. the consumer, the public) not give the artist income for life. Once an adequate incentive is provided for the creator-type, the copyright has served its purpose. You want patents to be allowed to be sat on forever and ever?
DING! We have a winner.
THIS is why it is different from your house. Your house is NOT about adding to the common store of art and science. The ONLY basis - and the explicitly cited basis - for copyright and patent protection is to add to a shared heritage.