It's because there ARE contributions from the owner, even a passive owner has what I called "dividends from stored labor": they took on the risk, created a functioning company solving a problem, did the work of organizing the systems/structure that run the thing, and so they are entitled to a piece of the revenue as well (as that revenue couldn't have been earned without their contributions either).
My point was that owners currently get a disproportionately large piece, and that the gap between what owners get vs what workers get has been widening with time. And not only is it widening, but it is doing so while the relative contributions to the business' success are often narrowing (give the huge increases in worker productivity in most domains).
It's of course hard to precisely quantify these things, but the worsening gap between worker and owner incomes is quite obviously not proportional to any change in how much value owners add to a corporation compared to workers (if anything, workers' contribution has been increasing, not decreasing, as productivity tools and techniques have been advancing).