Some do [0]. But obviously the gov & corporations controlling them prefer a more expensive prophylactic than a dirt cheap one.
0 - https://chrismasterjohnphd.com/covid-19/povidone-iodine-kill...
0 - https://chrismasterjohnphd.com/covid-19/povidone-iodine-kill...
edit: less sketchy link to a study of Iota-Carrageenan nasal spray for covid prophylaxis: https://www.medrxiv.org/content/10.1101/2021.04.13.21255409v... not peer reviewed, but a registered clinical trial reporting positive results.