The reason is that Strongman loses by the cooperation. Originally he saved 10 hours by cooperating, but under the new assumptions, he loses 10 hours. Building his own house all by himself will require only 70 hours of work, but if he divides the job as assumed in Table 5, then he must work 80 hours. Feebler on the other hand would make a preposterous gain. Whereas when working alone he would have to work 205 hours, he now needs to work only 90 hours.
Cooperation now loses mutuality.
Strongman refuses to cooperate, simply because he is logically selfish and the transaction causes him a loss. He refuses to be an altruist. He is to be commended for his selfishness. Of course not. If the two of us are to cooperate you will have to pound all the nails, but you will have to do some of the sawing.