I think the OP is asking why Debian doesn't do it automatically. It can't just remove the old kernel immediately, because what if the new one fails to boot or otherwise doesn't work? It's good to keep an older kernel around so you can go back to it if necessary. In theory, they could implement some logic that automatically removes older kernels while keeping the latest one or two untouched.

But generally I agree it's the user's responsibility to make sure they have enough disk space available and to clean up when necessary.