You're underestimating the impact of combination: those startups will need CFOs and council and so forth and having attached professional schools with their own robust alumni network(s) helps.
Which is one reason why companies in those respective fields tend to recruit from those schools. I'd be highly surprised for example, if Baker & McKenzie spent a lot of time at the University of Virginia recruiting law students, but would fully expect Google to spend lots of time harassing M.I.T. students (but not Harvard students).
Also, not all of the top schools have representative schools of business or law. And for those schools that do, I'm curious how much interaction there is among the three student groups (technical, law, business).
What does happen is that later on substantial #s of your classmates while wind up having having gone back to the law school or the b-school or whatnot.
This means that even if you, personally, never met anyone at the b-school, you probably know people who went there later on, making most b-school people only a couple degrees of separation away from you.
This means the big alumni network is loosely connected, but is comparatively connected, and predictably so; it's also the case that probably someone in your undergraduate class went to Wharton (or whatever) and thus probably there's some chain-of-acquaintances connecting you to the class at Wharton some year, but it's a lot more tenuous than the intra-university network.
So, short version: not that much interaction @ the time of study; across time, the alumni bodies are much more cross-linked (though admittedly not to the same extent that you see inside the particular sub-schools).