Steam provides much more than just a store. Having to print, burn, deliver and sell physical CDs is not cheap. Nor is the service Steam provides that physical discs wont. Also, did you pass over the fact that PS, Xbox, Nintendo, Apple and Google also have charged 30% for their online stores?
Anyone can go get free games at Epic and be happy they save 100% cash on their games. and that is why Epic's user base grew 173% withing last 6 years but with 1.6% revenue growth. If this doesn't make you realize that we pay for a good service, then nothing will.