The practice of taking pools of capital (derived from pension funds and very rich people) and deploying it in funds to acquire ownership of companies—often by taking public companies private and running them better in the hopes of selling them off for a significant profit—is going from strength to strength. Private-asset managers raised a record of nearly $750 billion globally last year, according to McKinsey and Co. That included a jump in so-called megafunds, which contain more than $5...