It's because people ended up with models that were thousands of lines and difficult to reason about. Out of curiosity, did you end up running into this issue and how did you deal with it?
I work on a few projects that do have a model that is over a thousand lines long. A lot of times as the model gets more complex, you start moving associated model logic into their own models, which helps reduce the problem space. I think its fine because the logic ends up being cohesive and explicit. Whereas services end up with logic being hard to track down when they get very large and usually scattered.
