Because most offers are "subject to favorable financing" and if the person doesnt get favorable financing, the offer falls thru and the seller ends up holding the housing longer w/o a seller and paying the carry. People will often sell at a lower absolute price (slightly lower) for all-cash offers without the issues of financing.

Also, financing can fall thru for silly reasons (eg the bank found some crazy lien from the 1940s) and want it cleared before they offer financing.