You just answered your own question - they don't (yet?) have such a feature.
And it's by far not the only airline without one - there are lots an lots of airlines that do not have it, or have very serious conditions on its usage (only available on certain routes etc.). Small regional airlines, carriers in developing countries with no large internet penetration, even some LCCs (in addition to W6/WU at least IV and DY spring to mind) just to name a few examples.