It wouldn't just give him access to melee-only items, but also make him more efficient with some other items and runes.
This item's passive is just 50% effective if you are ranged.
And Rakan actually never was supposed to build tank. He was Riot's first try to create a support champion with an assassin playstyle. Meaning that he was supposed to build squishy, rush into the enemy team to do his thing (in his case, instead of killing, his thing was supposed to CC the enemy and use his Q to get the healing for an ally) and then immediately escape again with his E.
A tank would be supposed to stay in the middle of the team and keep ccing them. That's also why Rakan gets fairly good AP ratios, to encourage him building enchanter items like:

Also to note how even his passive shield scales with AP. If he would be supposed to build tank items, then his passive shield would scale with defensive stats like health. But it doesn't, because again he is supposed to build squishy.
For a good reason. Rakan already has a great amount of safety in his kite with several dashes, shields, healing, movementspeed buff and hard CC. Escaping as Rakan really isn't hard and killing that guy when he builds full tank is even more frustrating.
His entire design actually screams that in your face. He is a battle dancer. Do you expect a charming dancer to be tanky? No, they are supposed to be elusive, but fragile. Exactly what they wanted Rakan to be. Building squishy, go in and do stuff, but escape before the enemy retaliates. Like a dancer, weaving in and out of fights. Not staying in and take the beating.