The nonprofit shell exists because the founders did not want to answer to shareholders. If you answer to shareholders, you may have a legal fiduciary responsibility to sell out to high bidder. They wanted to avoid this.
Anyway, in a strict nonprofit, the proceeds of a for-profit conversion involves a liquidation where usually the proceeds must go to some other nonprofit or a trust or endowment of some sort.
Example would be a Catholic hospital sell out. The proceeds go to the treasury of the local nonprofit Catholic dioceses. The buyers and the hospital executives do not get any money. Optionally, the new for-profit hospital could hold some of the proceeds in a charitable trust or endowment governed by an independent board.
So it's not as simple as just paying tax on a sale because the cash has to remain in kind of a nonprofit form.
I am not an accountant either and obviously there are experts who probably can poke holes in this.