It probably costs more to do that than to maintain a constant temperature. When you turn the setpoint down, everything inside your house and also your house itself starts losing heat. When you turn the setpoint back up, the cooled off house and items inside of it will suck up most of the heat until the stuff is warm again, and then the air warms up.

This is much more noticeable when you go into a freezing cold building and turn on the thermostat, it takes almost an entire day to heat up the stuff and building.