It depends on the item and for important reasons. The limitations of the auction house chief among them, with the requirement that you sell either singles or a full stack. A lot of items that stack 12 would be a pain to collect and sell as stacks of 99 and people may not want that many. So on a univeral scale, I cant agree this would be a "QOL improvement."

Certain items should stack 99 and I believe there was an official thread for suggestions on this. If you have specifics in mind, you should say so, but it absolutely should not be universal.