What makes solder harden?
Solder on a circuit board is just as soft as the wire solder it came from, since it's the same material.  However, the wire solder is not supported by anything, so feels much more bendable.  Note that softness isn't the same as bendableness.  Wire solder can also feel softer to something like pinching with your fingernails because most solder is hollow with a soft flux core, and you are collapsing it by pinching.

Solder on a circuit board is usually a thin layer that is well supported by a the board itself thru a thin layer of copper, and pins of whatever device is being soldered down.  This makes it feel a lot harder than the unsupported wire.