Because that's literally what previous games from both respective franchises allowed you to do (Hell, even GT7 STILL allows you to do it as long as you have the credits). At no point has anyone asked for a system that takes so painfully long to get to a point just to where someone could finally change an Exhaust (and funny enough, people has already found an exploit just to basically make the grind far more bearable). Having watched gameplay, its easily the most tedious bit of grinding I've seen and I say this as someone who has experienced some serious grind fests. Maybe you don't seem as bothered but as countless gameplay videos and screenshots appear to show, that's not the case for so many.
As for "ignoring the reasoning", many have said that on paper it sounded good but its the execution that has rubbed so many the wrong way (a familiar tone that is echoed with GT7 in General). Its not the idea that's the problem, its the end result repeatedly shown that is the issue.