Fedora [2], Ubuntu [3], and FreeBSD [4] build .NET from source themselves. A lot of work has been done to make it possible to build .NET from source [5] without closed source components, so it might just be a matter of someone being motivated to create the package for Debian.
[1]: https://learn.microsoft.com/en-us/dotnet/core/install/linux-...
[2]: https://src.fedoraproject.org/rpms/dotnet8.0
[3]: https://launchpad.net/ubuntu/+source/dotnet8
[4]: https://github.com/freebsd/freebsd-ports/tree/main/lang/dotn...