Because gold efficiency can be very misleading if you aren't careful. Especially if you are using it as a measure of the quality of the item.
You are missing out on the fact that we don't know the "true" cost efficiency of most items because most passives cannot be estimated the way other stats are. There is also the inequality of passives, there is a great variance in the strength of the items. The passive may be a significant part of the cost of the item or it may not be. Sometimes it's the reason you buy the item, sometimes it's only a free gift with purchase.
If you really think its the passive, question if that nearly 110% gold efficiency (compared to ~85-90% for most other crit items) has anything to do with it.
And this is a problem when you have completely missed that on LoL wiki, unlike the other Zeal items, Runaans has part of it's passive factored into it's gold efficiency. If you remove that from the calculation, Runaan's drops to sub-100% gold efficiency like the rest of them.
Same with Morellonomicon for mages. People buy it because its so insanely cost-effective, and grants AP+CDR. They don't even buy it for the passive.
Everyone buys it because with the nerfing of Athene's, there are no other CDR-Mana options.
And of course they don't buy it for the passive. That's just a bonus. It isn't useful in most situations which is why it doesn't weigh heavily in the item cost.