Because it is a subtle restriction. Many players don't understand that is how the mechanics work.
What about a hard density restriction? If your realm has less than 3 nobles per region, you just simply cannot initiate a TO. That would make more competition for positions and make moving to less dense realms more attractive.
We already have the hard density restriction. it is at 1.8 at the moment.
I don't know why you want to push for hard restrictions. We have more than one subtle restriction. Forming a super large realm is not efficient. It is not just gold. You have to constantly manage your regions. Not many people like running around spamming courtier work.
