One of the reasons resistance should make you less tanky is the abundance of true damage and pen.

There hasn't been a significant number of people building full resistance items, think banner, zzrot, locket and stoneplate, as first items for a while. When they were viable enough to be built first (in the case of zzrrot or banner) it was because of the champion playing them had an unhealthy gimmick with them, or because the item active was too good.

Nowadays these items can't realistically be built until third anyway due to lack of hp, and cannot be effectively built because of the excessive amount of true damage and pen in the game right now.

With the popularity of champions that have true damage and pen increasing (Darius, fiora, gp, Camille), tanks can't build items that give only resists anymore, especially early on, unless the active is brokenly powerful (stoneplate, though fiora will totally ignore that you bought it anyway).

The shift away from health makes it even more effective of a purchase, meaning people will try to stack as much of it as possible on their resists, leaving less space for these resistance items, especially when stoneplate exists.

So, if we wanted to make resistance items better on their own, true damage and pen must be reduced.