Munich's train connection is not really good. Nearly no long distance trains and a very long travel time to Munich main station where you need to change unless Munich is your final destination. So you lose over an hour, nearly 2 taking the notorious unreliability of German trains these days into account.
Frankfurt is not a pleasant airport but at least it has a long-distance, high-speed train station.
For the most people destination is likely to be Munich , it is urban center with large population and density .
People don’t think long distance trains when talking about commuting to airports, usual expectation of any urban airport is it well connected to the city center
Have you been to Frankfurt airport long distance train station? It's pretty busy. That many airports have worse connections doesn't mean doing better would be unnecessary.
I more or less regularly have to use Munich or Frankfurt airport and take a long distance train. Flying closer to to/from my destination in Germany would be possible in theory, but is not attractive, neither by price nor schedule options. So I have the choice between a horrible airport with a good train station and a more decent airport with time-wasting train connections.
Frankfurt is not a pleasant airport but at least it has a long-distance, high-speed train station.