The assets being in the game is easy. Coding all of that to show up in certain situations for certain eras, having players designed for one area of the game showing up in other, and all of those types of things are more difficult than one would think. I don’t know how their game is setup, but I do know there is so much coding done and it’s added up over the years with legacy code going back to many 2ks ago.
When adding or fixing features, they would have to check so many areas of the game to make sure a change/fix/new feature didn’t unintentionally break something else. Using CFB25 as an example, they made a bunch of quality of life fixes in their first patch, but unintentionally broke the game score bug and a major area of the recruiting screen. Took them at least another month to patch that and then broke a few more things. So it really depends on how they have it setup and written and how much they can add without breaking what’s already working in game.