Why do people think a browser is safe to use? Why do people think any app is safe to use? Why do people think a website is safe to use? Why do people think an OS is safe to use? Why do people think a driver is safe to use? Why do people think a firmware is safe to use? Why do people think a hardware device is safe to use? Why do people think the chips inside are safe to use? Why do people think an ISP is safe to use?

If you have a point to make about it being particularly unsafe or different from any other internet/software trust, make that point. Otherwise you know well enough that there isn't any other option but trust, and people generally trust stuff until given a good reason not to.

The point is that Safari's extension system requires using the App Store, not that it's inherently safer. In some ways, the "App" model that Safari uses could be more unsafe, regardless of Apple's code review.

Nonetheless, a critical engagement of software "safety" would require another few thousand words, at least.

Security experts.