Save files are not just memory dumps, you need to have a serialisation/deserialisation mechanism for the game state. But this is not generally a viable way to do RTS multiplayer, so you either need to build both a save game system and a deterministic replay system (which might each make the other more complicated), or go for the latter which you need to do anyway and then reuse it for saving the game as well.

Factorio manages to pull it off and I'd argue they are tracking far more individual entities.

It's not impossible, it's just a question of if it's worth the effort. Factorio was kind of forced into it, because a) they built a save system before they went multiplayer, and b) it's impractical to replay a factorio save when loading a game, because it lasts a lot longer.

(note that factorio does a hybrid: multiplayer works like an RTS but starting from a save game instead of from scratch. If your PC can barely keep up with a server that you're joining, it can take a long time to actually get into the game or actually just not be possible)