.NET Framework just get security updates means no rebuild, no unnecessary updates for your users
And as phillipcarter noted, increasingly more libraries drop 'netstandard2.0' target, depending on the workload the performance difference is going to be anything between 50% and multiple orders of magnitude (e.g. Regex engine has gotten tens of thousands of percents faster for some patterns).
Working with .NET Framework from non-Windows environment requires using Parallels / UTM on macOS or QEMU or any other VM env. on Linux - there's Mono but it's not a replacement, and I don't think Visual Studio will work under Wine.
There are all sorts of hidden costs that you don't plan for, that come with using .NET Framework.
Even if Microsoft puts in a huge effort to updating a migration tool to be seamless, we wouldn't use it. It'll only happen when either the larger enterprise corporation migrates, or when Microsoft forces people off 4.8.
It's close to the change when get/settwrs were made mostly redunant (but not quite).
Just so much less boiler plate that you didn't even really realize you were writing.
Most of the time boring old technology beats shiny new one.
However, the workplaces that adamantly refuse to upgrade also select for developers who may not be familiar with pattern matching or other features at all, and never read release notes.
Yes... if you do self-contained deployments or build containers, then of course you're tied to the version you've built with - but that is your choice and therefore your responsibility.
You are now blaming having that some deployment choices cannot be updated manually. It is the wrong mindset.