I don't know, there's something about it, the familiarity? Maybe it's comforting? But there's something about a mug of regular everyday tea. I'm a brit abroad and get people to bring me over bog standard boxes of the stuff. Of course there's tea here and I enjoy different varieties. It's not the cost, there's just nothing like a nice unpretentious mug of tea that nothing else beats.
Maybe it's a social thing, you can't go wrong with it, "want a tea?" "sure", "milk sugar?" Although I don't know why anyone asks if you want milk because if you refuse it, you'll get a funny look. It's just done everywhere. It's just not done to answer to tea, "yeah, sure, earl grey, lightly steeped with a splash of lemon.".