Besides savings, its more to do with skill as well... Having the right products isn't gauranteed to give you good results e.g. Using a clay bar wrongly can mar your paint, an ultimate compound by hand improperly can do this to you car Ultimate Compound messed up my paint .
Not that you can't do it yourself and its rocket science but you need to have the commitment to learn it end to end and be knowledgable of the do's and donts + the products/equipment. Buying that all for doing your car on off is going to expensive with compound/polishing/clay products sitting on the shelf for a long time unused. For busy people not into the passion of detailing but wanting to have a detailed clean protected car, they should look into spending some money (which is less than 1% of what the car costs 1-1.5million average) and you are basically improving your car's look/state by 50-95% (which in turn is also increasing your resale value so its a win win). One you get it detailed, keep the maintenance products and then maintain it yourself.
Just my perspective though, YMMV...