There's the whole tea ceremony but they are drinking tea continually, they don't have time to be doing this all the time, every time they drink tea.

They will use proper tea though even in the thermos. Most tea is not expensive (when you consider how little you use) and making it from leaves is really not that difficult, I'm not sure why bags became so popular. It's even cheaper to buy the stuff loose. I know tea can get stupid expensive, but the stuff the average person drinks every day isn't. And it's still good tea.

But if you are brewing strong black tea and dousing it with milk, I guess it probably isn't going to make that much difference, and I guess there is an element of convenience, and not needing a teapot. Most tea in the UK is made directly in the mug with a bag these days.