There is no market for it to provide the ROI. If there was a potential market, even at the expense of college, they would have done it. There have been multiple efforts to create a new sport market in major sports (e.g., USFL, ABA, CBA) and they all failed, despite the willingness of investors at the time to make large investments.
The minor leagues are not a profit center for MLB. They are a business expense of MLB. They cover most of the expenses for minor league teams in exchange for the right to place their players on those teams.
MLB would make money in the short term withdrawing from the minor league system and letting independent minor leagues and the college game develop their talent for them. But beyond just the tradition, the MLB views in-house player development as something worth spending on.
There is no question that letting college basketball (and Europe, and independent minor basketball leagues like the old CBA) do their player development work for them is the most cost-effective option for the NBA. And if that were what the NBA league office was saying, that would be the end of it. But instead they are pouring money into the D-League and openly discussing shouldering more of that burden themselves. Your disagreement is with Adam Silver, not me.