I say it is more akin to shipping produce than TVs. We don't get the majority of our veggies from a single farm for all of the US.
As for why mattresses are so expensive, I think a lot of it is "because they can." I think there is also materials and such. A bed with two people in it might hold over 400 pounds for 8 hours a night 7 days a week for years. If a tv had to do that it might be more expensive too.
I lean toward the 'because they can' argument, which means, what the market will bear.
People want to spend on mattresses because they believe spending a lot on mattresses will improve their lives in some way. Less fatigue, less pain, less stress, etc. These are big selling points. However, they are selling mostly snake oil.