Brake parts

Now being new here and not searching this entire site, I have a question for yall about doing a disc brake conversion. ......... Why is everybody buying old used rusty brake rotors and calipers to do a conversion when you can still git um new or rebuilt at your local auto parts??? Seems like all you need as far as used parts would be the control arms, spindles and the dust or backing plates. Just being curious, I called and ask for rotors and calipers for a 75 Duster. No problem. Said they could have um there in one or two days. Yeah, thier aftermarket but does that really matter? Your already not gonna have an all original car by converting its brakes in the first place. The used rotors out there that people are offering for sale as a conversion at top dollar I might add, have most likely already been replaced and are aftermarkets and rebuilds as well.

???????? Robert ?????????