Why are bots clicking on my ads?

To steal your money.
A very large number of websites are being run by criminals. These websites, like most websites on the internet, use ads to generate revenue. These ads are served from the same ad networks you use to advertise your service or products.
Instead of waiting for humans to click on the ads, the criminals use bots. These bots are sophisticated, they imitate humans, and are difficult to detect. Most of these bots are able to bypass the ad networks' bot detection systems.
Your ads are appearing on these criminals' websites, and they're being clicked on by bots. The ad networks charge you for this, and the criminals get paid a commisson - roughly 60% of the money.
Click fraud steals at least $100 billion from advertisers every year.
To cheat KPIs.
There are unscrupulous media agencies who manage the ad spends of large advertisers. These agencies may be listed companies with stellar reputations, but their business model is a scam.
After receiving their clients' marketing budgets, they set up pay-per-click ads on legitimate ad networks. Everything appears normal.
Behind the scenes, the media agencies transfer some, or all, of their clients' money to shell companies, and use bots to click on the ads. These fake clicks simulate real visitors, allowing the media agencies to hit their traffic KPIs.
Some of these media agencies even use third party bot detection services, selected for their outdated technology such as IP address blocking, which is easily bypassed by modern bots. This helps maintain the media agencies' air of integrity, while ripping off their clients.