Problem is that the minimum fee for 24hr target could change a lot in an hour or less because some news story happened or simply when enough people noticed a lull in fees and decided to move their fees when it's still cheap. In that event 24hrs coild easily turn to days.

Wallet developers and payment portals gets a lot of flak for overcharging fees, but ultimately it is still preferrable to using a conservative estimate and deal with the likely support call when the payment fails to confirm, and there is nothing they can do but watch it get bounced between nodes.