Debian only ships free software (in main, but that's a detail).
This is actually enforced and there is processes in place to ensure that it stays that way.
This means that all new software that Debian packages is audited by a group of volunteers, the ftp-masters team, they check copyright, license and stuff like that.
If all binaries in Debian would vendor all of their dependencies, this would cause a lot extra and duplicated work for the ftp-masters, a team that already have a lot to do.
Same with security, if a popular go library needs to be patched to fix a security problem, then it's easier to do that in one place instead of patching it in N different binary packages.
This is actually enforced and there is processes in place to ensure that it stays that way.
This means that all new software that Debian packages is audited by a group of volunteers, the ftp-masters team, they check copyright, license and stuff like that.
If all binaries in Debian would vendor all of their dependencies, this would cause a lot extra and duplicated work for the ftp-masters, a team that already have a lot to do.
Same with security, if a popular go library needs to be patched to fix a security problem, then it's easier to do that in one place instead of patching it in N different binary packages.