I think it's just because supply-chain attacks are not common enough / their attack surfaces not large enough to be worth the dev time... yet...

Sneak in a malicious browser extension that breaks the permissions sandbox, and you have hundreds of thousands to millions of users as an attack surface.

Make a malicious VSCode/IDE extension and maybe you hit some hundreds or thousands of devs, a couple of smaller companies, and probably can get on some infosec blogs...

The solution at my job is you can only install extensions vetted by IT and updates are significantly delayed. Works well enough but sucks if you want one that isn't available inside the firewall.