Nobody does. Not twice anyway. The resulting brew tastes thin and underdone, no matter how well the water may have boiled in the nuclear oven. There simply is no way to make good tea in a microwave. Boiling the water to pour over leaves, be they in a bag or strainer, or even in a pot, doesn't help. I've idly wondered why for years. After all it'd be so much more convenient.
Because a proper cup of black tea must be made with water that’s come to a rolling boil... Heat at the bottom of the kettle... The hot water rises and the cool water falls in a cyclical fashion, which uniformly heats the contents of the kettle to a boil ... But microwaves don’t heat water evenly, so the boiling process is difficult to control.
Having surmised as much myself in the days when I used to heat milk for younger children – the top of the milk would be scalding, and the lower depths lukewarm at best – I'd once or twice tried stirring the cup of boiling water to see if that helped. Not a bit. But now I know why.
When tea leaves meet hot water, hundreds of different compounds that contribute flavor and aroma dissolve and become suspended in the water. Black tea contains two kinds of complex phenolic molecules... These are responsible for the color and the astringent, brisk taste of brewed black tea, and they are extracted only at near-boiling temperatures.
Water also cooks certain volatile compounds, chemically altering them to produce more nuanced flavors and aromas, such as the earthy, malty, and tobacco notes in black tea. When the water isn’t hot enough to instigate these reactions and produce these bold flavors, tea tastes insipid.
If that's not bad enough, superheating the water, which is always a risk with microwave cooking, can destroy the oxygen normally found in water. "The longer water boils, the more dissolved oxygen it loses—and tea experts say that dissolved oxygen is crucial for a bright and refreshing brew."
There's more, but by now I've heard enough, not just about the perils of nuking tea, but of reheating soup in the microwave. I gave up on that particular fool's errand many years ago, preferring to use a stovetop pot. Or rather, not preferring to, but choosing to because there seems no way to ensure a uniform heat when nuking a bowl of leftover soup from the fridge. I guess what happens is that significant chunks within the soup retain their chill, making the whole exercise pointless.