I don't understand how Unsellable house profit split is fair

I could be wrong or missing something, I'm not great with numbers and when we bought our house we didn't have to deal with a mortgage or anything, so I don't have experience in real estate.

How is it fair that the twins get half of the profit the sellers make? I understand recouping the money they spent on reno and I also understand them taking some profit. But 50%? Throwing 60-70k into a renovation doesn't entitle them to 50% of the profits. What am I missing? It feels like the sellers would be better off taking a loan out and doing their own renovation.

Edit. Some people have kindly informed me that they don't take 50% of the total profits. They only take 50% of profits from the adjusted price. That makes much more sense to me and I can see why a homeowner would do it