Because 77F is the desired temperature
Let's suppose you have an envelope for the $800 rent due this month. Add money til it's $800. Not til it's $801, because $800 is just right.
The system is viewing temperature the same way. Temperature == 77F, just right, nothing to do.
Anyway, the system should have hysteresis
You don't even want the thermostat hitting 77.1 degrees, going "Too hot, run A/C" and running it for 20 seconds until it's 76.9 degrees. That would result in something called short cycling which is rather bad for air conditioning units.
Not knowing your system*, the thermostat doesn't want to risk a short cycle, so it will intentionally add some hysteresis to the cycle. For instance it may fire off at 79F, and run until 75F. That assures the unit isn't running too often.
Regardless, the command temperature is the satiety temperature; ideally if actual temperature == command temperature, it should not start a heat or cool cycle.
If 77F is the "unbearable" temperature at which you want the A/C to come on, then set your thermostat to your "comfort" temperature e.g. 74 or 75. Adjust to suit.
*many systems are "oversized" in the sense that in normal conditions they have more than plenty of capacity; however on very hot days that also means they have enough capacity.