In a nutshell because that's all that Boeing provided for the certification package.
Seriously, though, certification is an expensive business. Unless there is either a general demand for a facility amongst the buyers (ie the facility is provided or the lack depresses demand for the product generally), or the OEM foresees a marketing opportunity in providing an option, it's not going to happen, regardless of whether it is feasible or not.
This is no different to what a mouse trap manufacturer provides in the package on the supermarket shelf.
Boeing is in the game to make a dollar for its shareholders, not play altruistic provider of freebies.