I love f droid and wish there was some way that all those apps could get links for iOS too (an iOS index?) - given an option I go for f droid and any iOS equivalent available always
The reasons for the absence are pretty self-explanatory. iOS itself is proprietary. First, someone would need to initiate a project to create a free software clone of iOS, like ReactOS re Windows or even GNU re UNIX. That's a worthwhile project in and of itself.
That's not a reason at all.