Why do technological monopolies only last for a short period of time?

All the technological monopolies have obtained patent rights for the products which is only for a short period of time. Beyond that time there are various substitutes and imitated products in the market similar to original products.

Also technological monopolies set higher prices for their products but the substitutes are priced lower so customers choose to take substitutes when they are in the market.

Also the technology becomes obsolete very quickly when there is a new technology. In the current market scenario the technology keeps upgrading day by day. When new technology arrives the old ones become obsolete.