Just a guess, but because the base of the toilet needs to support users of all body weights, a sturdy material like porcelain (or clay) is used. The tank lid is likely poured and fired at the same time as the base so that they match. Agreed that it makes for a darn heavy obstacle to get at the mechanism. And to all who have posted about having tinnitus, I feel your pain. My left ear has been affected for two years now and I've had to learn to live with it. I hope you all fare better.