Gossh thhat was SO LONG AGO too lol - ya'd think they'd fix that by now.
This option doesn't exist because when auto-stacking existed, it resulted in numerous item duplication exploits. For example, if someone was in the bank trying to deposit a stack of 10 pots onto a stack of 15 pots, and someone else withdrew the stack of 15, you occasionally ended up with 25 pots in the bank as well as 15 pots in the second player's inventory. To alleviate this, auto-stacking the guild bank was disabled, and each "stack" is counted as a separate item. There may be something they could do to get around that, but I don't know what that could be.