Comment by orbital-decay

Nobody locks the games down. Most games with highly active modding scenes were never supposed to be modded, they all had to be reverse engineered or the source leaked. No tools, no engine modifications, nothing. DOOM, GTA, Mario 64, STALKER, Minecraft, the list goes on and on. Games like TES, ArmA, Quake, anything Source/GoldSource based are exceptions. And even then all major Bethesda games are heavily reverse engineered, the Morrowind engine got rewritten basically from scratch and the other games have a ton of fixes and sideloaded code. There's even an entire vehicle simulator (!) strapped on top of Fallout: New Vegas, which is absolutely insane if you know anything about its engine.

It all comes down to two things, player interest and the range of expertise/amount of work and coordination required for your mod to fit the base game. For example STALKER SoC has a lot more story mods than, say, Skyrim, because SoC is pretty janky: it has few animations and voiceovers are reserved to key plot moments, so it doesn't take much to match the quality.