Assuming they only got as far as their residency (and didn't end up as attending physician), it's possible that they didn't see themselves spending a full 7 years as a resident doctor (making under $100k/year working 80+ hour weeks) only to spend the rest of their lives doing more of the same except with a much higher salary. If they already graduated their residency then the reasoning is the same except it's be a much harder decision because of the sunk cost.