Re: Third party
Same reason that if you're running .NET in AWS Lamda's you don't contact Microsoft for support, you contact AWS
There are now 2 "supported" options to install dotnet on Ubuntu
1) Through packages.microsoft.com (compiled by Microsoft, supported by Microsoft)
2) Through packages.ubuntu.com (compiled by Canonical, supported by Canonical)
If you'd rather read the information on Canonical's site then you can find it here - https://canonical.com/blog/install-dotnet-on-ubuntu