something i can not understand, a why not, in relation to y stability from x axis

why not have rails or fix round bar, on both the top and bottom of the x axis, IE, 2* 125mm box, 2200mm long, why put rails jujst on the top and rely on waight and engineering to get a stable point for Z to fix to.

It would seem an obvoius solution but i see its not used, that means these something am clearly not accounting for. am prity sure i could get them level to each other easly, so what am i not thinking of ?