It's all about financial incentives and this far down the pipeline it ain't happening. GTA Online is a multibillion dollar business, GTA V Singleplayer not so much.
One more longterm succesfull open-source sim of that kind without online shop and half the industry is forever gone.
[1]: For example here: https://www.gtaboom.com/gta-online-update-dlc-history/
I will likely buy next GTA on launch if it launches at same time on PC as consoles.