> How is this scarier than any other software you
    > use but haven't seen the source code to?

The only code on my computer that I do not have the source code to are the JetBrains IDEs and Kerbal Space Program. With both of those companies I have a financial relationship, and both those companies earned my trust, and their software I have to download and install explicitly.

Contrast with webassembly where there is no trust between the entity running the code on my computer, no realistic way for the average user to consent to having code run on their system, and no reason for them to earn trust.