Why are input resistors in power supplies covered with shrink tubes?
It's because the resistor is intended to be the "weak link" in case something in the power supply fails shorted -- in other words, a cheap fuse. It also serves to limit the inrush current. The resilient heat-shrink tubing is there to contain any fragments resulting from its (possibly violent) failure, and to prevent the free ends from moving around and creating additional shorts.