1. Because they are cheap and nasty.. .2. There's little competition. APUs are usually SFE * so the aircraft manufacturer is the customer rather than the end user.
* SFE = Seller Furnished Equipment. There is no choice of supplier, you get what comes with the aircraft. As opposed to BFE or Buyer Furnished Equipment where the customer selects from the available options and negotiates price and product support terms with the equipment vendor.
In some cases the APU in one model of aircraft is the engine core or gas generator of the Prime Mover in a smaller aircraft. Since the reliability of the small aircraft engine is then as expected for a main power plant, this seems to imply that lower quality standards do indeed apply to auxiliary equipment. Which confirms point number one above.
**********************************. .Through difficulties to the cinema