If milk curdles when left out of fridge why does it not curdle when added to tea or coffee?

Im not positively sure but i think it is because it mixed with another liquid or mixture and when they come together the milk almost fades away as it mixes with the other liquid. And i think it is because it is not alone it the cup, it has something else with it so it won't crust up or evaporate or whatever its called so quickly!!! xD