Because very often the reason you're selling a house is that you're buying a house, and so you need 100% of the value of the sale to go towards your new deposit

Because very often the reason you're selling a house is that you're buying a house, and so you need 100% of the value of the sale to go towards your new deposit. It's no good having 75% of the cash now, you need it all as a deposit and the mortgage company isn't going to take the risk that that 25% turns up eventually.