Is the lack of this item intentional Riot?
 

Is there a reason why there isn't a magic damage version of blade of the ruined king for champs that DPS with magic damage?

There is an item that does that

Liandra's Torment deals (5%health) or (10% if slowed) to enemy champs per spell you strike them with.

It builds out of haunting guise

I was thinking about something like this.

70 AP
10% life steal

Unique Passive: Your attacks deal 4% of targets current health in magic damage and heals you for half that amount.

Unique Active: Drains target champion dealing 150 magic damage plus 87% of your AP and healing you for same amount. Additionally you steal 30% of their movement speeds for 2 seconds. (60 second cool down)

That sounds like a combination of existing items: Mask, Will, Fidd's Drain, and Malphite's Shard. It sounds rather odd, and potentially broken.

So, the exact same item except technically not and on a completely different type of champion that doesn't mesh with it mechanically.

I'm afraid not, no.

If you're utilizing auto-attacks with an on-hit build in the first place what's so bad about the AD?

Because your be using magic penetration which does not help AD or the on hit effect at all since they are both physical damage.

You're doing a bigger chunk of damage as Physical, why would you build magic penetration?

I have the distinct impression that Blade of the Ruined King was put in as a means of making an item NOT for "on-hit" builds like Madred's Bloodrazor wound up being so often; it wound up being too simply: just take a Wit's End and Madred's to get both major damage and tankiness to prevent counter-bursting, and then the Malady would ensure they softened up.

Remember that the BotRK (I guess "bork") is relatively UNIQUE in that its on-hit effect applies lifesteal; your lifesteal is not going to apply to magic on-hit damage.

There are several champs that is not true for.