Because they do not create heat.
They direct heat that arrived from the sun into a concentrated, smaller surface.
But the heat they direct, already passed through the atmosphere and would just be spread out on a larger surface if the mirrors did not exist. The total heat input into the earth atmosphere is unchanged, only distributed differently.
The facilities modify the albedo causing light energy that would have been reflected to stay inside the system. That said, the numbers are so ridiculous low that aren’t even worthy of consideration (just like the stadium seating half-seat comparison on human CO2 intervention, etc).