Trucks just plain wear them out, If you get 1 million miles out of a mechanical speedo set up you are doing good, Electronics are good to 2 million or more (never owned a truck I put 2 million on). bushing can wear, thrust washers can wear, all change the alignment of stuff. Not rocket science, but more watchmaker, jewellry maker kind of work, if you have to make your own parts.
Many cars use a one piece cable which can transmit thrust force to the head, many truck set ups use a floating cable end at the speedo to prevent that from happening.
Point is: it is possable to make a long lasting cable drive speedo, but most cars aren't made that way and need parts after 30 years, you are on borrowed time. On the plus side, there are those that restore them and can make it good again, just not as many as there were 40 years ago.