It is done by amateurs who design "super secret encryption algorithms" that nearly always turn out to be trivially breakable. I'll assume you don't mean this "

It is done by amateurs who design "super secret encryption algorithms" that nearly always turn out to be trivially breakable. I'll assume you don't mean this "random algorithm" you "throw in" to be secret, (otherwise see the other response) and instead you hope that complicating the algorithm to make cryptanalysis more difficult. The thing is - You don't want cryptanalysis to be difficult! You want the world's cryptographic experts to look at your algorithm, immediately see the potential ways to crack it, and quickly find that none of them actually work. A complex encryption algorithm is bad because there could be vulnerabilities hiding in the complexity. That "swapping every 7 symbol" could make it easier to crack by creating subtle patterns in the output.