Because most public web sites, especially with ads, execute untrusted code on your local device, requiring firmware, OS, browser security sandbox and web-page access control contortions for constantly-attacked web browser engines.

e.g. the most recent iOS 15.3.1 security update for iPhone 6S and later devices, https://support.apple.com/en-us/HT213093

> WebKit: Processing maliciously crafted web content may lead to arbitrary code execution. Apple is aware of a report that this issue may have been actively exploited.