Why is the milk used in latte steamed and not boiled?
A latte is similar to a cappuccino but with different proportions. It's unsurprising that a similar method and equipment would be used. Cappuccino needs steamed milk to get the froth, so a coffee bar machine has a steam pipe. To boil the milk for a latte would require an extra piece of equipment and the associated space. 

Is perfectly possible to make an espresso-based drink with milk heated on the stove or in a microwave, that would be a close approximation to a latte -- possibly indistinguishable. But if you boil the milk, it will change the flavour.