The social fabric of the system would fall apart if people could trivially buy top level domains. Even the not so trivial buying that ICANN began in 2015 in order to profiteer was a serious mistake (to everyone besides ICANN).

it would be a global first come first serve namespace. Everything conceivable would be registered. Only the diversity of TLDs and SLDs saves the current DNS from this. In fact ICANN's money-grubbing TLD spam actually improved this, since there are now far too many TLDs to squat them all.