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Achievements were added June2016 so anyone who owned the game beforehand and either didn't play again, or always played with achievements disabled (often due to certain world settings or with mods) wouldn't have them obtained no matter how easy they were.
Some other games that have achievements that SHOULD be 100%, such as "start the game" are often at 74-95%, even lower if achievements came in at a later date.
Achievements were added specifically for the steam release, so as long as steam players are involved, achievements were there from the start.
The real reason is, that far from all people play (or even finish) the games they bought.
Check achievement for different kind of games, and it rarely goes over 20% of people.
Will it be possible to buy the expansion DRM-free on factorio.com then register it on Steam, like we did back in 2016?
Yes, as long as steam supports it for DLC (I hope so).