Sure, but a lot of Business undergraduate programs, even at prestigious Universities, are now "pre-MBA" and very MBA-focused, if not "direct to MBA" and allow taking bare minimums of non-Business classes and just about guarantee MBA program entry. For MD this sort of "academic incest" makes sense that you are going to have more because there is too much specialized knowledge to learn during graduate programs. (But also most pre-Med doesn't pre-qualify Med School like "pre-MBA" can.) JDs still seem to expect a variety of candidates of different undergraduate backgrounds, though "Pre-Law" sometimes exists, it often isn't a specific "program" and to my understanding can be several different options from very different undergraduate college options; "Pre-Law" seems as much about navigating the analysis paralysis of all the possible paths as anything else, without narrowing the number of paths.

I think the MBA programs have built "pre-MBA" programs not because they have so many skills to specialize, and not necessarily because they have so many possible paths to try to navigate, but because the it sells more Business school undergraduate credits.

Good MBA programs still exist. Not all MBAs involve "academic incest", and there are still MBA programs that encourage non-Business undergraduate degrees. Not all "academic incest" is bad either. But there's definitely an anecdotal sense that many of the people I see with MBAs spent the least time learning anything that wasn't taught in a Business School classroom, with the least consequences for their non-Business School GPAs, because the Business School wants that graduate degree funnel and the tuition dollars it guarantees, than any other graduate degree program I've seen. (Hence why I mentioned "perverse incentives", especially. The Business School wants you to do well in Business School so you keep paying the Business School. The Business School cares less what you do outside the Business School so that you keep paying the Business School.)