The substance of keeping the rating consistent has to do with how the plane operates, the flight controls, etc.
There are potentially perverse incentives here, but it's not as trivial as that.
The FAA says you have to have a B-737 rating to fly any 737, regardless of revision [1]. The MAX series is the maximum number of changes the FAA would permit and still allow it to be called a 737. In theory, someone who is type rated and familiar with the -100 series should be able to safely operate the MAX 8.
Most likely the airlines have rules above and beyond that (like requiring sim or right seat time in a particular revision), but this is the motivation behind the aircraft. If they had made a 738, it would have needed to go through certification as if it were a new design. By making it a revision, the certification process is significantly reduced in both time and costs.
1. https://registry.faa.gov/TypeRatings/ (note the MAX isn't present on this list; I assume due to it being grounded)
Maybe a pilot can chime in on what the real training is like when switching between aircraft of the same type rating and how a manufacturer or the FAA decide on what constitutes a new type.
Original 737:
https://en.wikipedia.org/wiki/Boeing_737#/media/File:D-ABEA_...
Max:
https://en.wikipedia.org/wiki/Boeing_737_MAX#/media/File:HS-...
The fuselage, tail, wings and control systems are close enough to the original type to stay certified.
Bad: "Taxis have to meet these standards."
Good: "If you pick up strangers off the street and drive them somewhere for money, locating them by visual signals, you have to meet these standards."
Bad: "If your plane is a new model number, it has to re-certify."
Good: "If your plane exposes an interface to the pilot with incompatible expectations from your existing models, it has to re-certify."
(Not making any claim about whether aviation or taxi law meets this standard, just making the general point.)
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