You'd be surprised. Advertisers wouldn't be running the ads if they didn't have positive ROI (that is, the value they get from the ads is larger than what they paid for them). A good ROI is usually at least 5x (that is, for every dollar the advertiser spends on ads, they make five dollars). Google's estimates are even higher at 8x:

we conservatively estimate that for every $1 a business spends on Google Ads, they receive $8 in profit through Google Search and Ads.

(source: https://economicimpact.google/methodology/)

Retargeting ads - the ones where you view a product somewhere then see ads for it on Google, Facebook, etc - are especially successful. They have a very good clickthrough rate since the product is already something the user expressed interest in.