Because once your vehicle is out of warranty they want it to break so you go buy a new one.
Even with full synthetic oil do not go beyond 8000 miles between oil changes – PERIOD. The best oil in the world is clean and most oil filters do not filter out particles less than 10-12 microns which act as abrasives in your engine.
Do not go passed 5000 miles between oil changes unless you are using a premium oil filter such as a Wix, Bosch, Mann, Mobil One, or K&N which are designed to hold 8-15,000 miles of dirt. The difference in quality between a premium filter vs an economy filter is significant.
Most oil change express shops use cheap $2 oil filters even when you buy the $90 synthetic oil change. If you must use an oil change shop your best bet is to buy your own synthetic oil and filter and pay the labor to have your oil and filter installed.
NOTE – While some advanced synthetics can go 15,000 miles before thermal breakdown, that dirt that the oil filter cannot remove is still there.
Change your coolant every 3-4 years. Coolant after that time starts to go through “electrolysis” – a nice way of saying it turns acidic and eats your components from the inside out. Go 100,000 miles before changing your coolant and you WILL be replacing components you would not have had to.
Change your transmission fluid every 60,000 miles and only use premium fluids. IF you want it to last 400,000 miles that is how to do it. If you live in a hot climate area or tow you should seriously consider adding an aftermarket transmission oil cooler.
Change your brake fluid and power steering fluid when it starts to get discolored such as turning brown. NOTE – Dodge, Ford, Mazda, Toyota, Lexus, Nissan, Infinity and some other brands all use transmission fluid for the power steering – DO NOT use “power steering fluid” in such vehicles. If you do not change these fluids in a timely manner the internal components WILL corrode.
Change your differential fluid every 30-50k, on the early side if severe service and use PREMIUM differential oils such as Lucas, Mobile One, or Royal Purple for example.
If your vehicle is forced into deep water change all of the fluids as soon as possible.
Lastly, only a fool buys cheap brake pads, but that is another article.