Because the stuff that is currently working is a known quantity. I can think of VERY little aftermarket/replacement auto parts made today, whether US, Mexico, China or Canada, that I have ANY confidence that they are made anywhere near as good as they were OEM, or even aftermarket 30 years back. Poorer machining, thinner/lower quality materials, more plastic, you name it. The brand doesn't matter either. I recall buying the highly recommended Spicer/Raybestos Pro Grade ball joints for the 10 bolt, and seeing machining chips left in the joint when I went to install the boot. Griffin radiator failures. Mis-made 14SF axle seals. I'm sure I could come up with more examples if I try.
Run what is working until it fails, then replace with what you think will be the best quality component.