The point is that, while these blocks can be complicated and need lots of customization, the ability to grab the source and modify it yourself is a huge enabler. If I could grab a stock IP from a repo and then spend 40 man-hours customizing it, it would be a huge benefit over spending 1000 man-hours making the IP from scratch.
I hate to appeal to expertise, but I've written HDL, synthesized it, and done mask layouts before. I understand that it requires a lot of customizing, but the first step to all of this is sharing. Sharing infrastructural components is a huge reason of why the commodity software business is as robust as it is.
And let's be honest, a state of the art block ported to the latest process isn't going to be 40 man-hours to customize, you're probably looking at more like a couple man-years unless it's very well designed... Which is why reusable core shops are a viable business. And how vibrant of an open source community would you have, if it took a small team a year just to customize & implement the open source design to their application? You would have very few users.
Now, you could argue that because a lot of the real work is in the customization that they could open source the core. But where does the customization end, and the core, the root functionality begin?
I think a more realistic pitch would be an open source testbench or behavioral model that verifies your core is fully spec compliant.