Because it's goes same as here - majority of ppl doesn't support increasing of party size.
It's quite literally the most requested feature on this forum. dwarfing everyone else. The dedicated thread doubles in size all the others. And a second thread on the same topic is STILL among the ten most popular in this section.
Fun fact: that second thread started advocating that a party of 4 was better but unlike the first most of the posts in it argue AGAINST the original thesis.
Even my personal crusade about the current party controls comes only second, and by a long shot. Which is ironic in some sense, because it's probably basically a prerequisite to make a party of six viable.
Anyway, this is why we ask people to do a search before posting and to go back to other popular threads, because this discussion
is wasting everyone's time making most of us repeat over and over arguments we already had more than once.