Because if the board is oxidized you may well blow the parts right off the board... (don't ask me how I know ;)).
So use some pressure but don't overdo it, you won't get the last out anyway like this that will happen during the drying phase but at least you'll get the bulk of it.
Also: there may be a lot of very subtle damage that will only show up later: moisture pulled into wires due to capillary action, switch contact corrosion (inside the switches, so invisible), contamination by stuff dissolved in the water, mechanical damage due to swelling and so on. Water damage is pretty nasty.
So use some pressure but don't overdo it, you won't get the last out anyway like this that will happen during the drying phase but at least you'll get the bulk of it.
Also: there may be a lot of very subtle damage that will only show up later: moisture pulled into wires due to capillary action, switch contact corrosion (inside the switches, so invisible), contamination by stuff dissolved in the water, mechanical damage due to swelling and so on. Water damage is pretty nasty.