Because the existence of billionaires is predicated on the exploitation of human labor and unsustainable environmental harm. That level of wealth hoarding is harmful to economies, as it reduces the amount of money in circulation. No one person, no family, could ever conceivably even SPEND a billion dollars anyway, and it is inherently immoral to accumulate wealth so narrowly while so much of the world lives in abject poverty.
Better then to create a wealth ceiling, a point at which all wealth over a certain point is taxed at or very near 100% to incentivize people to actually spend their money rather than hoard it, stimulating the economy and bettering the lives of far more people. Better even still to create and regulate economic systems that protect workers and the environment in a way that such extreme levels of wealth accumulation aren’t even feasible.