It's not all that surprising if you think about it. Let's say you go right to college after HS, and take a year off between college and med school (due to the competitiveness this is becoming very common, maybe even more common than direct admits). You'll enter medical school around age 23 and will be there until around age 27. You'll be a resident until at least 30, but maybe as late as 33 or 34 depending on the specialty - family med is only 3 years but some of the more intense surgical residencies can be 5-6 years. Fellowship is optionally another 1-4 years on top of that. With a few notable exceptions, the most lucrative specialties have you not drawing more than a token paycheck until your
late 30's, best case.
Combine that 23ish-30ish age range with the fact that many (probably most) residents will work above the ACGME-mandated 80 hours a week for at least the first half of their residency, and I'm honestly shocked it's only ~21% of physicians that marry other physicians. Not to mention the fact that the income bump is substantial.
My fiancé is a physician and we met when she was a resident, and I'd say about 40% of all the residents I met during her residency who were in relationships were in a relationship with another physician.