Following on from 2. if you focus on the skills to do the backend you are never going to learn or keep up to the date with all the intricacies of browsers quirks. Vice versa if you spend all your time dealing with browser quirks you may not pick the best architecture.
It's not that we aren't capable of doing either, it's more that you can't be amazing at both at once.
However I, personally, would add one more.
#6 You can't be a ninja at both front and backend
Following on from 2. if you focus on the skills to do the backend you are never going to learn or keep up to the date with all the intricacies of browsers quirks. Vice versa if you spend all your time dealing with browser quirks you may not pick the best architecture.
It's not that we aren't capable of doing either, it's more that you can't be amazing at both at once.