You underestimate how little money such a dlc makes. It’s the same reason why the promised SP addon for GTA 5 was cancelled. Return of investment is magnitudes higher for any GTA online content. Making a new car model is little effort and brings in millions of dollars.

Having to deal with story, level design, voice actors and possibly animators or even Mocap: soooooo much more work. Sure they could do it, and even Earn good money with it. But online pay dlcs are so little effort for so much more payout , why even bother?

Rockstar hated what FiveM did, even threatening them at the start because they were afraid it would compete with their own online service. And now they bought them up, but only after realizing how much more people ( or rather their wallets) they can possibly reach.