Photoshop mockups are static not dynamic.
You cannot interact with it.
You cannot click the links or enter text in the form.
It does not behave they way your design will behave in the browser.
Designing your websites in photoshop wastes time.
First you design in photoshop, then you do it for the browser.
Why not just do it for the browser at once?
Add in the factor of switching between the millions of tools in photoshop to get a simple thing done.
You cannot collaborate with your back end and front end developers.
You set up GIT or dropbox and you change the HTML/CSS, it changes for everyone else instantly.
Photoshop is form over function.
You need form and function.
HTML/CSS lest you build a usable real world product.
Photoshop gives you art that is lovely to stare at but does absolutely nothing.