TV, radio and newspaper ads are regulated by the FTC. Advertising on these mediums is limited (24 hours in a day, X number of pages in a paper) and illegal content can be easily tracked and archived, and prosecuted.
Big Tech serves an ungodly number of ads from all kinds of scrupulous and unscrupulous parties. Ad content is almost impossible to track and monitor. Big Tech says "let us self-regulate" while also saying "it's not possible to ensure thaf every single ad we serve complies with the law, we're doing our best".