This app was mentioned in 3 comments, with an average of 1.67 upvotes
That's just because iOS has terrible app support for open source projects like this. Get Android there are a bunch of Doom engines updated regularly some free some $1-2.
https://play.google.com/store/apps/details?id=com.kokak.DoomGLES
https://play.google.com/store/apps/details?id=net.nullsum.freedoom
https://play.google.com/store/apps/details?id=com.opentouchgaming.deltatouch