Unity frustrated the flying fuck out of me. i mean it's great for people who want to drag and drop and google snippets but fuck I felt so powerless. It makes you go through so many hoops if you want to do things like optimization, or using other low-level stuff. On top of that I found the editor impractical, the library to be garbage and skinny, and I said it before but it's the most important key point: powerless.
In Unity if I had an issue in my code that didn't really originate from my own code (e.g. an exception deeper down in the engine originating from a call of mine) debugging became a hell. They don't give you the source code unless you go on one of their expensive licenses. With Unreal (being fully free, no restrictions aside from royalties) I can drop a debug breakpoint, find out why it crashes and move on. It's great, I love it and it gives me the power I needed.
Another point is how Unreal moves forward. They invest a LOT into R&D and every major update has features that actually are groundbreaking sometimes. They focus on performance at the same time, so you get a great mix. Low-end users can play just fine because of the optimization and high-end users can benefit from all the post-processing and rendering techniques that they've added. I'm bad at writing shaders too so I'd be clueless when it comes to adding bloom or post-processed AO.
On top of that, and yes you can pretty much make both look equal, I find that Unreal's shaders are better. More to finetune, more options.
I do have to say that Unreal isn't as mature despite it being UE4 - I believe it was a full rewrite instead of built ontop of UDK so there are things that don't work or bugs that occur. Then again I'm sure Unity would have those too albeit probably less often.
SOOOO yeah, that's why