1) In many legislations level flight upon reaching the MDA is forbidden, for quite some time.
2) On some installations (such as the type the OP is most familiar with) it is the technological limitation of FMS which needs to have MAPt coded at RWY THR with a hard constraint of 50' TCH in order to properly calculate the VNAV path. Especially the segment below MDA.
I think the 1) is actually quite obvious, called CDFA. Sorry for being blunt.