Technically, you can on an iPhone too, just not for free and not anywhere near as conveniently. There were various tools in circulation while there were no decent Jailbreaks. One that springs to mind was iReSign - according to its github page it's for re-signing app bundles with enterprise keys, but that's not what it was used for back then. People captured the .ipa files and signed them with developer keys and all sorts of unfortunate things happened after that.

TL;DR: "sideloading" for iOS = Free: no. Convenient: no. Possible: yes. Out of reach of most people: absolutely yes.