Why are smartphones portrait?

I've noticed it seems impossible for the same website to work ok on a normal computer and mobile. In fact most websites on mobile browsers are broken or horrible to use. I'm currently going through the process of trying to make my website work 'ok' on mobile.

The biggest problem I find is that computers display in landscape and smartphones usually display in portrait, thus making any layout/styling useless cross platform but also the organisation of the pages themselves may not be ideal on both platforms.

But why is this!? It seems to be the biggest issue fragmenting these two platforms. Are they like that just because apple thought it would be cool when they made the iphone?