Wouldn't you then want ten times the number of people eyeballing the code? And boatloads of free contributers submitting change requests to fix potential problems?
And a marketplace of users that can freely choose deciciding which "IP (I still find the term so deeply offensive, I can't help but it I have to put it in quote)?
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.