Because auto companies have recognized that vehicles have become dispensable... Once they make the sale, they want your service money, and the more the better.... Think about it, if they teach you that you only have to change the oil every 7500 or 10,000 miles because your using synthetic guess what, synthetic oil does nothing to remove the carbon build up or foreign material, and if you change your oil 10 - 15 times in 100,000 miles, your engine is junk by then.....
"Sorry Sir, your engine has failed, we'll put in a new one for ya - will be about $5-10K.
There is no way around it, and people buy into the idea that auto companies have been pushing and selling them for a long time.
"With Synthetic you don't need to change your oil as often." Bullspit
It called marketing.....
Change your oil regularly and often....
My regiment -
500 miles, then every 3000.
I might start moving towards every 5000 though.