My joke was that people still use Flachspüler for their originally-intended purpose in the late 19th Century, which was to check their stool for pork-borne parasites.
American toilets aren't filled to the top, only about ⅓ of the total volume is water. The idea is to have enough water to keep fecal matter completely submerged, to cut down on odor and to prevent skidmarks. However, older toilets are indeed wasteful, using about 14.25L of water per flush. The law was changed in 1994 to limit newly-manufactured toilets to 6L per flush, but the first generation of "low-flow" toilets were so ineffective that consumers simply refused to buy them, and most homes have a toilet made in 1993 or before. Only in the last ~10 years have modern "high efficiency" toilets taken root, which let you choose either a 3L or 4L flush. Those, too, are still designed so there's plenty of water to cover stool.

Ultimately, the most pressing issue with toilets is not the water height, it's that each one should also have a bidet, but too few actually do. Personally, I'd like one of those Japanese toilets that has an integrated heated bidet and plays music.