Why can't i say "tea is enough"? Like when you get offered "do you want tea, shall i make some coffee, or maybe a Cocktail?"
I'm speculating here, and could be entirely wrong. In your suggested scenario, a different Czech sentence might be needed. This one is literally saying, "there is enough OF tea," meaning sufficient tea is available at this time. In your scenario, the meaning would be along the lines of "Tea is sufficient for my immediate purpose of drinking something, I don't need anything other than that." (But, like I said... pure speculation on my part.)