I want to stress that I actually don't give a shit (I am not that into tea, I just drink it sometimes, it's not a big deal to me), but I am curious as to why.
I think the theory is that "stale" water lacks sufficient oxygen for tea brewing to fully brew or something.
I'm not a big tea drinker, but I am definitely not a super fan of water that's been sitting out a day. There's not been time for stuff to grow in it yet, but it still tastes gross.