But GW appears to churning out models for basic humans for every other faction and game, except for Imperial Guard.

It's financial imo. Someone at GW is trying to work out a roughly stable year by year release window to maximise the impact of their main releases. There have already been some similarities with what's been announced this year to 2019 (which is also why I'm predicting "Saga of the Beast" will be a SW vs Ork replacement for the Shadowspear box in March) but more importantly Imperial Guard make sense as the major release in August to fill the gap left by the SM Codex + Primaris marines.

If they weren't planning a major "splash" release to generate big sales I can see absolutely no reason the faction would have been left for so long.