It's usually terrible advertisers. They've got their account set to show to everyone in a super broad range. Like "uses a phone, under 50, in country X" and that's all they're going by.

This is combined with Google's shitty "we'll tell you how much to spend and who to spend it on, trust us!" Automation and dark patterns, which just spaffs all your money in places you don't want to. Such as mobile apps! Which used to be one click to disable but now its 200+! Or location, which now defaults to "people who have shown interest in your country" when it used to just show ads to people in the country. Or keyword matching which used to be a lot more strict, but now they keep broadening things. (One headphone company we worked with spent thousands on "telephone" keywords, that they never entered into their account)