let's pump the breaks a little bit
#1 - some components / parts are almost impossible to get - many of the large distributors have issues obtaining inventory ( (one example : QBP) and therefore it can be significant challenge for the bike shops to acquire the parts ; many bike shops have resorted to searching and sourcing parts from Amazon and eBay
#2 - individual chainrings can be a challenge to find (depending on model) and can be relatively expensive if they are available - so often it can make more sense to replace the entire (complete) crankset