That's because they do not mount and dismount - they are treated as foot in some circumstances, and as mounted in others.
People keep trying to treat them the same as dismounting mounted troops (!) - but they are not - the ruels are different although some aspects of them are similar.
for example elements of mounts (ie riderless horses + holder) - mounted troops who dismount have these, and so can mounted infantry, but they do 2 differetn things.
For hte mounted troops they allow remounting to become mounted again - but for the infantry they allow faster move in some circumstances - they are never used for mounting or dismounting in the way of cavalry (or Kn, etc).
The trick to understanding mounted infantry is to realise that they are NOT dismounted cavalry - they can be depicted similarly, but behave differently. the ruels are under the same heading, but they are different for each!