Why no "custom" roms, like Android phones?

i apologize silly question, coming android , i'm pretty new on iphone hacks / jailbreaks. on htc android phone, used play custom roms time. on android, once 'rooted', practically device. htc provide updates os, , devs hack updates (adding / removing components it) , provide custom update packages ready flashed device.

why isn't such thing done iphone? there technical reason on why no 1 has ever devised way modify apple updates, add baked in root access , cydia, , flash on top of jailbroken iphone? not saying @ easy, can't wonder why it's done time on androids not on iphones.

 

whited00r.com

though older iphones, , custom have jailbreaking.

 

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