Apple is providing a service (yea there's no alternatives on iOS but still)... for 30% they take care off all your payments from all countries, they take care of refunds, dealing with credit card companies, and for the most part dealing with customers in general.
They also provide the market, the installation system, the update/upgrade system. Having tried to write some of those myself and the servers to maintain them I can tell you I don't personally want to run them.
Compare to Steam. Steam is also 30% on a platform you can do it all yourself if you want. Sure they provide the most exposure but similarly they also provide most of the other services that make it worthwhile.
Assuming you could do that do you really want to do all that yourself? Think about it. If you did it yourself you'd need to run a server to put your software on. Keep it secure, updating it constantly with security patches. Make a relationship with a payment processor. Handle refunds yourself. Figure out an upgrade system. Figure out how to deal with failed installations. I'm sure others could name many more things you'd have to do.
30% doesn't seem unreasonable to me given it would take more more than 30% of my time (and therefore money) to do this myself. At least that's my perspective.