Budgeting and launch costs are probably the biggest reasons. Kepler, for example, has already used up its budget to build, launch, and operate the telescope fo

Budgeting and launch costs are probably the biggest reasons. Kepler, for example, has already used up its budget to build, launch, and operate the telescope for several years. Even if free replacement hardware was available there may not be enough money to launch it. Also, the way most spacecraft are built is as one off prototypes, so building another copy, even if all the spare parts were available, is a very non-trivial endeavor. Even if it didn't take as long as building the original it would still be fairly expensive just to go through all of the necessary testing and integration steps. The end result of all of this is that it rarely makes sense to spend the time and money to refly a spacecraft and instead it usually makes more sense to simply forge ahead and seek funding for a follow-on mission of a new design. Here's a few case studies. The inaugural launch of the Ariane 5 rocket failed, which resulted in the loss of 4 "Cluster" spacecraft which were designed to study Earth's magnetosphere (a loss of about $370 million in hardware). However, ESA decided to fund replacements and they built 4 new satellites, making use of flight spares and extra hardware to save on costs, and then launched the new satellites on 2 separate Soyuz-Fregat flights. However, it still cost almost as much money to launch Cluster II as for the originals, so this was a rather heroic effort on ESA's part. In 1999 the Wide Field Infrared Explorer (WIRE) spacecraft was launched, shortly after launch there was a malfunction that caused a loss of the main science instrument on the orbital telescope. The mission was not replaced. However, the loss of WIRE caused other IR survey mission proposals to receive greater interest, which led to the funding of the "Next Generation Sky Survey" Medium-class Explorer mission. This infrared full-sky survey mission was launched in 2009 (as WISE, the Wide-field Infrared Survey Explorer) and achieved all of the science goals of the original WIRE mission. In 1993 all contact was lost with the Mars Observer spacecraft right at the moment of planned orbital insertion. This loss led to a re-evaluation of the unmanned Mars exploration program at NASA and eventually led to around 10 new missions to Mars over the following 2 decades, most of them successful.