Tablets vs. Phones : Price

Today, I woke up with a simple question, it may seem stupid but it really bothers me...

Why are tablets cheaper than phones ?

Seriously, specs are usually better than phones, or at least on par, screens are better, with better resolution. Battery is also bigger. It is not only an Android question, as the iPad is also cheaper than an iPhone.

Is it only a marketing question or is there any spec/cost reason ?

The only reason I could find is that manufacturing all the elements in a smaller frame (for a phone) may cost more, but I have no idea wether this argument is valid or not.

Thanks for your answers !