Are they, though?
If they were, then tracking users via third-party cookies and other resources wouldn't be possible. Nor would it be possible for a web site in my browser to suddenly start taking up all of my CPU/RAM due to a programming error or malicious site such as a crypto-miner. For the relatively little isolation that does happen, sandbox-escape vulnerabilities seem to be getting discovered all the time.
Also, as a technical user, I want more control over what web sites can do with my computer than a non-technical user might.
The more holes you poke in a sandbox, the worse a sandbox it is.
Probably because there's no way to say that it "doesn't put any of the user's data at risk". WebGL has been abused for browser fingerprinting which itself puts user's privacy at risk, but it also has a long history of very nasty vulnerabilities and exploits. It's been fully disabled in my browser for years because of the security issues.
WebGL is one of the biggest fingerprinting vectors on the modern Web platform, and expands browser attack surface significantly. Most webpages should absolutely not have access to privileges like this.