It is hard to find employees/co-founders that (a) have the right skills, (b) the right interests, (c) the right industry knowledge/contacts, and (d) are in the same place in their life to make the same amount of commitments.
And even when you do you find such people, it is hard to give them 10% equity in the beginning and then tell them that they are under-performing, and only deserves 2.5% equity.
Now, I agree, that the definition of under-performing is very different for a startup compared to a more established company. I have also found people are willing to accept when they are under-performing, but contracts and equity that is given is harder to change.
However, I would love to have the right cofounder, and even share the equivalent equity with him if he can take over half my burden.
Perhaps the answer is to find people with a very good fit, start them with low equity (~1%), and tell them how they can get more equity. And then doubling equity multiple times as they are able to rise to a founder level responsibility.