Because they are laptops. I can't even use my 16" MacBook Pro on the couch. It's portable sure, but it's not a laptop. You can take it anyway, only move it from desk to desk. It's the single heaviest thing in my bag when I travel.

I can do the same work on a MacBook Air when I'm away, and it's basically just a desktop when I'm home. To me it would make way more sense to have a desktop at home and a 12 - 14" laptop, if it wasn't for the cost of having both.