And what if you gathered all of these in one device?
This is pretty much the entire point of tablets, along with convenience. I'd rather carry one, light device with multiple functions than multiple devices with single purposes.
Also, if you aren't sitting down, you literally cannot use a laptop (without one of those convoluted shoulder holder things I've mentioned) and even then, 90% of trackpads suck ass and you'd need extra room for a mouse.