Because most merchants would then need to borrow money to pay their suppliers and expenses while awaiting payment, which would end up costing them a lot more than an extra 1%.
Why 'on month'? You'd get a 1% discount at most; if you 'buy' that discount by freezing all funds for the whole chargeback period (90-120 days) then it's 3-4% APR.
[edit] the thinking is, if your revenue is $100/year, then in (A) scenario with higher rate you pay $1/year more in fees; in (B) scenario with delayed funds you need a permanent loan of ~$25 which will cost you more than that unless you can get an APR of lower than 4%.