The problem is that it costs money to develop iOS apps. Even just the $100/year or whatever it is, is enough to severely discourage open sourcing your apps.

On Android, you can create apps for free in your spare time and distribute them however you want. The lower barrier to entry means a lot more open source hobby projects.

Distribution is another big issue. Even if you found an open source iOS app, how are you going to run it on your own device? You can only run apps blessed by apple or pay the developer fees to compile it yourself.

Apple severely restricts the hobby/ open source market from both ends.