In addition to what others have mentions, the returns are typically low as well. Going off what others said, since Google Adsense ads are targeted, that means

In addition to what others have mentions, the returns are typically low as well. Going off what others said, since Google Adsense ads are targeted, that means your competitor would hypothetically be advertising on your own site, and second, startups usually are in niche industries where the bid prices for ads are low. Unless the startup is in the business of legal help for mesothelioma, they can realistically expect a dime or two per click on ad. Consider an ideal 2% CTR (click through rate), and you'd need more than 5000 visitors to pull in only about $20.