You can do many things via jailbreaking but there are some things you cannot do, too. But iOS was not designed to be Jailbroken and there are no custom ROMS. What you do have is the capability to do many--perhaps most--of the things a custom ROM provides without the need to change ROMS or worry about ROMS in the first place. Many that point to this lack of ROMS are largely clueless.
If there were a need/demand for custom ROMS, we would probably have them. Or perhaps there are a few; I no longer look past Cydia because problems await you when dealing with apps from untrusted sources.
Rooting or Jailbreaking is something most users could care less about until they hear about it and then they decide they need it. I jailbreak for one specific reason: the ability to access doc and text files on my iPad. I do use themes and a few tweaks, like adding more icons to the bottom of the screen; custom arranging my icons, adding screen widgets and other things.
You should see the looks on people when I show them my themes. Some people have asked me about my Android Tablet . . ."Gee, that locks like an iPad" because it runs DreamBoard and an Android theme. It works exactly like an Android tablet, too.
Trouble starts with the tech wonks arguing the minutia that might not even matter to those of us that are deep into Jailbreaking.
Remember, sometimes, you can add all of the features you want from Cydia that duplicate some of the features offered by a custom ROM on the Android side.
Jailbreaking adds lots of capability to iPads and iPhones. I never saw the need for a custom iOS ROM because everything I want to do is easy to do with a tweak or three.
The lack of a custom iOS ROM is not a reason to not like iOS devices until you know for a fact that you cannot do something with a jailbroken iOS device. No offense, but from your post, it is clear you might not know what you can do with a jailbroken iOS device and a trip to Cydia or a repo.
I might suggest that you can actually do more with a jailbroken iOS device than is possible with an Android device. The numbers of themes, mods, tweaks and other stuff available through Cydia and perhaps 500 plus repositories is astounding. If you want things for the Android side, you are "stuck" with whatever ROM(s) are available as well as the apps designed for rooted devices.
There are tens of thousands of things available on Cydia and there are vast numbers of repos outside of Cydia, so we have tens of thousands of potential "markets" that let you do anything you want or need to do.