If so, why just not use native desktop/mobile video editor app instead?
How you would check that web app, which JavaScript/WebAssembly source/binary should be firstly loaded into your browser from 3rd-party website, would not "install malware or spyware or do anything else malicious"?
Sandboxee was released in 2004, and there have been various bits of registry journaing/snapshot software since the late 1990's which combined with NT ACLs could provide at least as much security as your getting out of a browser sandbox.
Browsers have an absolutely huge sandbox surface and lack even the most basic of fine-grained security controls out of the box. Enabling even basic restrictions to avoid fingerprinting has turned out to be difficult. How you do you know (particularly with firefox) that one tab isn't actively picking out data from another tab open to your banking site. Much less the problem with malicious javascript being injected via the ad frameworks.
Browser security is pretty much a joke.
The level of technical know-how necessary to accomplish that is astronomical compared to "here's a link," especially since it's cross-platform out of the box. That's why it's a no-brainer: any sane person is going to prefer that over installing untrusted software. Basically zero users know about the sandboxing solutions available for their system, much less willing put in the effort to Do It Right.
Just few examples of "pretty secure modern browsers nowadays".[0,1]
[0] https://bugs.chromium.org/p/chromium/issues/list?can=1&q=lab...
[1] https://techdows.com/2020/05/mozilla-halts-firefox-76-rollou...
So many of the projects I see mentioned here are neat hacks, but that's really about it.
People writing a complete operating system for an 8 bit micro made of discrete chips? That's neat, but not really useful, and most people won't ever take the time to boot it once.
An FPGA that runs web assembly code? That's nifty, but if we want high performance we probably won't use web assembly to do it.
You invented a completely new language that has all the features of C but none of the things that are annoying? We'll add it to the list of C-like-but-not-annoying languages.
Found a great new way to estimate how much capital you should try to get when creating a start-up to sell your all-natural soap products in California? That's great for you... a little bit niche for the rest of us.
You re-implemented emacs in python? That's a really impressive technical achievement. No one will ever surpass you in either emacs implementation or python hacks. Also, no one will ever use it.
Same thing with Hackaday... lots and lots of "cool" projects where neither the creator nor the site mods seem to realize something is just a nifty project, and not the salvation of the world.
Pure interest and sublime pointlessness are, of course, two of the classic hacker motivations.
Edit: https://news.ycombinator.com/item?id=23246221 is happily near the top of the front page as we speak.
By this I meant to include, for example, the Debian team. Of course there can be bugs/oversights, but the same is true of the browser solution. Is there even one instance of malware being distributed through the official channels of a mainstream Linux distro? Not that an in browser JS video editor isn't cool, but I don't see how it would have technical advantages over the various Linux desktop editors.
Many programs that don't need low level access like spreadsheets, pdf readers, word processors, image editors, etc.could be made into single page html files that would both be sandboxed and cross platform.
If you has access to internet you could also download & install native app. Also native app once installed could be used without internet access.