Except the sandbox is a huge target already, and breaking it means any website can now access and mess with your usb devices. If you can develop an exploit for Chrome's WebUSB system, you potentially have millions upon millions of targets available.

Downloading an arbitrary executable can be made safe (via multiple avenues: trust, anti virus software, audits, artifact signing, reproducible builds, etc) and once the software is vetted, it exposes (or it should at least) little to no attack vector during daily use.