Because the minds of the owning class that run these companies are twisted and only care about the Almighty Profit Incentive™

They don't care if their products are harming people because they don't care about you, the consumer, after you've bought the product. They got their profit out of you. Any bad press about it can be minimized with marketing campaigns to obscure the facts from the general, uninformed public. Prime example of this is Big Tobacco's efforts over the decades to hide the harmful effects of smoking before they could no longer keep a lid on it.

The censorship nonsense is surrounded by arbitrary, prudish culture bullshit which ad companies care about because they want to be able to market to the widest available audience, so they don't want to purchase ad space with platforms that permit anything controversial, thus platforms which rely on ad revenue for profit choose to enforce censorship so that those ad companies will choose to buy their ad space.

When asking "why do companies do [X]" it is always about profit.