Because that's how it feels. I appreciate the effort the guys at Bohemian Coding are putting into building a better tool for UI Designers, but I think we need to do more than just placing buttons and text.
Many people say Sketch is improving their workflow. Good for you. Now I'm going to let you know why it doesn't improve my workflow and I'm sure a lot more people feel the same.
First of all, where I work, I do more than web design. I need to provide content for the websites I design. That means I need to create animations or manipulate images.
There's nothing Sketch does that PS can't in terms of "designing". Multiple artboards and asset exporting is a nice touch, but I've been living without it and it doesn't bother me. Yeah, symbols are nice too, but I don't need them neither. So why using Sketch if PS works better for me?
I'm not trying to convince anyone to stop using Sketch, but there are a lot of Sketch users that try to convince me to stop using Photoshop and they're starting to sound like the Jehova witnesses of web design. In the end, the product you build doesn't say "Made in X Software", so who cares?
Use whatever software improves your workflow, because, that's the most important thing. Of course, with nowadays trends of creating flat designs in order to hunt likes on Dribbble, you can use MS Paint as well, so why using PS when you have lighter alternatives?
So all the best and keep using what you're using if it suits your needs! Forget about what "popular" designers say to use. You're not a pixel pusher.