Precious few? The entire PHP, JS/TS, and parts of the Python ecosystems are built around Semver. That’s millions of projects. Maybe you’ve just looked in the wrong place?
They are meaningless for projects like the linux kernel. The linux kernel has no stable kernel API and they aren't supposed to break userspace. There's not that much to semantically version without a stable external API.
Projects using semver try not to do breaking changes, which is why they don't change major versions often.
It works really well for libraries and APIs, but in my experience it doesn't work well for end-user-facing products. If you look at npm packages, a lot use semver.
The Linux kernel, for example, explicitly states that version numbers are meaningless.
In my experience, semver ends up just being more of a reason to never have a major release rather than a way to communicate breaking changes.
I don’t think Hugo uses semver anyway…