Because it was suggested over 3 years ago and it appears as though no one particularly cares for it (based on the number of likes which is typically the way to show support for a suggestion).
Interestingly, @BamaStangGuy posted there several years ago saying that it was a "must" to move from vBulletin to XenForo. To my knowledge, he probably has done more vBulletin to XF conversions without that feature... Hardly a "must" then