Right Sona is the archetypical enchanter, and Lux has some enchanter qualities as well they should be enchanting groups. Tanks should not have the same outputs that enchanters have this is important to what makes each class unique.
Tanks are designed to work best in team fights so groups of 3-5 makes sense. But if you are a tank using your ultimate to disengage the group fight then yes Zeke's will not be helpful, though if the other team is forcing you to use your ultimate to disengage then you are already on your back foot and any enchanter style team buffs are not going to be as impactful as your ultimate would have been.
Keep in mind we also just reduced ultimate cooldowns, this is a major buff to tanks, and Zeke's (so doubling down on a buff to tanks).
Edit: forgot to address MR items
MR items are very niche. Armor is always valuable. Armor helps vs Towers, Epic Monsters, and most enemy champions, whereas MR only helps vs certain champions. And a team of all, or even primarily, magic damage dealers is very rare. This means that we want a larger pool of Armor options to address those needs. Where as MR items are more niche due to their stats, but they have much more powerful effects to make them worth it.
Magic damage dealers also tend to focus a bit more on burst, and stacking MR items is already good at invalidating them, adding more just pushes that further. I think one more could be worth having, if we find the right effect, but I don't expect us to ever try to match the number of MR and Armor items.