In my experience, unit testing is more common in embedded C development, because the code is held to much higher standard of accountability.

Sloppy embedded code can kill people, or at the very least lead to incredibly expensive hardware recalls. With the exception of poorly thought-out privacy policies, it's unlikely that errors in a web app can cause that level of problems.

It's also much easier to deploy an update to a web app than a Mars rover or a car's brake controllers.