Yes, if you want to play it on Steam you need to buy the basegame there. However no one forces you to do that. I for instance has no interest in plying it on Steam as I can see no benefit to do that.
I can see one possible benefit to moving it over onto steam, and that is the modding community... do you know how much easier it is to just click subscribe... and the mod installs itself flawlessly on a majority of steam games... for the sims 4 you have to download each file and depending on what is modified, you have to place certain files in certain locations and if its wrong, it can crash the game and cause total chaos on the files possibly corrupting them, if steam did what they do for other game modding where you just click subscribe, it could do all the compatibility testing more easily and you would not have to manually drag and drop the modded files, possibly saving hours of manual labor.
