Part of what you're touching on is a general problem with rent-seeking, regulatory costs to entry, and monopolization in general, in the US at least. With physicians, this is kind of a consequence of a system their professional organizations have shaped: they kind of brought this ("this" being lack of buyers) on themselves to some extent by restricting supply.
But even outside of that, if you ignore that part of it, there's monopolization costs coming from other directions. Many physicians are getting squeezed by decreases in their options, as hospitals and clinics merge, which creates efficiencies that make it hard for small clinics to compete with, which in turn leads to fewer administrative options should a physician choose to try somsething else. My sense is it's even worse than that, because even if you still have a few large hospital systems to choose from, the administrators are being incentivized in the same ways across the chains, so it's one flavor of hyperprofitized healthcare corporation versus another flavor of hyperprofitized healthcare corporation. Basically, it's hard for small clinics to compete even if someone wants to, because healthcare has become a cuthroat profit-driven market, with lots of big money and plenty of opportunities for those in power to take advantage of advantages.
I guess what I'm saying is it's not just the debt for the young physician: it's a very risky thing to do, period, being a small clinic versus an oligopoly of large chains. "A doctor knows what his practice is worth and wants every cent he can get out of it" is one possibility, but another is that that seller is in a position of survivorship bias, and can't find buyers because they don't see it as a rational decision even outside of the debt question.
The reason why you don't see this with landscapers or plumbers or whatever is because those markets have far more competition, and they're not overregulated (at least to the same extent). Healthcare as a market is characterized by obstacles to competition everywhere: if you ignore the rationale for those obstacles, and just look at it for what it is, you're left with an infrastructure that's constituted largely of barriers and costs to entry and participation. Basically, the market has become too expensive even for practitioners to participate fully in, so the only thing left are financial heavyweights.