Because the switch has either failed or come off it's mounting. There is an outside chance, if there has been work around the handbrake lever/centre console, that the switch wires might have become trapped and could be shorting out.
it has a sticky handbrake switch
handbrake light
The handbrake light is also the 'low fluid' warning light. It is connected to the handbrake as that ensures that it is checked every time you use the handbrake. If it remains on all the time, then check the brake fluid and top up if required. If the fluid is ok then it is probably the switch on the lever and is adjustable.
Because you probably still have your handbrake on. If not, than you need to see the car-doctor.
it means your handbrake is on
usually a sticky switch on handbrake, had any welding recently ?, worn shoes ?
your switch that tells the car that the brake is off is defective
Release the handbrake
Either the switch (on the handbrake) is sticking or you have a short circuit, usually caused by corroded wires or connections.
Either your handbrake switch is broken and will not turn off or you have low brake fluid.
If the handbrake is not engaged, and the warning light is coming on, it may mean that there are components in the braking system that require maintanence. Or, it could mean that the master cylinder is low on brake fluid.
it means that the handbrake is on