A lighter oil in place of a thicker grease will gain you a fraction of lesser spinning resistance, at a whopping huge cost of the oil not staying in place as well, and possibly make the bearing more susceptible to contamination too. I.e. more maintenance/shorter life.
On a bicycle, where bearing drag is at best a marginal contributor to overall resistance, there's just not many people willing to do that trade-off.
But doable? Sure. I've even seen a track bike running with the seals/shields removed. Only being used indoors there wasn't much contamination to deal with, and maybe it did cut down on friction some tiny, tiny amount.