Why? .NET Core 1.x tried making everything a Nuget package and it was a disaster. ASP.NET is big which meant a huge number of dependencies, easy to create conflicts, long restore and build times.
It was much better to just include ASP.NET Core in the box with .NET and simplify it down to a single reference. And most people use the web SDK which does it for you so they don't even need that.
The other question is, why isn't what you posted the default, but defaulting to this <Project Sdk=...>? To me, it's a lot more intuitive, signaling that each project can have multiple framework references, vs. just one SDK attribute.
Make the simple easy. Make the complex possible.
From Python: have one way of doing things
From Go: explicit is bette than implicit.
Is Sdk=... is just a short cut to pull in a handful of framework references? Or are there actually more things happening under the hood?
Python really isn't a great choice for making your case. "Have one obvious way of doing things" has become an ironic meme since forever, and toolchaining and dependency management has traditionally been a hot mess (yes, even with pip and virtualenv, just not the total clusterfudge it was before. And don't get me started on pipenv.), and only gotten better recently with 3rd party tools like poetry, or even conda.
It's no different to e.g. create-react-app vs npm install react.