I don't even need a 4K monitor at £2000. A 15" or 17" external retina thunderbolt screen would be perfect. They already have the 15" panels, I wonder why they

I don't even need a 4K monitor at £2000. A 15" or 17" external retina thunderbolt screen would be perfect. They already have the 15" panels, I wonder why they don't sell them as external monitors too.