Because once you introduce those concepts in your world, nothing matters anymore and you can do whatever the fuck you want!
"We need to have our main character become evil but we can't, what do we do?" "Oh just make another version of him show up behind a bin or something"
"What if we had this same character...but as a girl?" "Brilliant idea! The fans will love it!"
"The world is in danger! The universe is going to blow up!" "Don't worry my friends! We have this other universe right here and the only difference between them is not important and may even be beneficial!"
"I'm you from the FUTURE!" "..." "You need to come with me and save the FUTURE!"
"Ok what the fuck is happening and why?" "Time is convoluted" "Bu-" "Shut up"
It kills my interest the instant it's brought up and i'm sick of it.