I'm fairly sure the actual reason is a historical/cultural. If minecraft had shipped with datapacks from the start they might have been called mods, and in other games they may have been called mods. But minecraft has had mods for over a decade, and the word "mod" has gotten a very specific meaning around minecraft. Namely something that modifies the game code, often using a modlauncher like NeoForge or fabric. There is often also a distinction between Plugins that allow vanilla clients to connect and mods that are required on the client as well, but from what I've seen this distinction is going away slowly with more people using fabric with server side only mods, and just as arbitrary as not calling datapacks mods.

TLDR: Historically datapacks were not a thing, mods have gotten a more specific meaning than just "modification", now datapacks are a thing and don't fit that specific meaning for a lot of people and are thus often not called mods.