While most champs have to balance out CDR and mana costs together, the champs this item excels on are those purely balanced around their CDRs. So while you can help to mitigate the problem that mana can cause (IE being completely unable to attack with spells) with items, no item besides the tear items really completely remove that weakness, and the tear items take significant time to build up.
The spear on the other hand completely mitigates the CDR weakness - simply auto and voila, 25% of your cooldowns are gone. Unless you are stunlocked for a literal 6 seconds, you will spike your power immensely with this item. There's no build-up, there's no ramping into this item - it's purchase and done.
Now we can argue that ER did the same thing and nobody was truly broken with that item - but the thing is that ER granted no bulk and restored mana on hit, meaning half of the stats and effects the item gave out were utterly wasted one these champs. To go ER rush would mean you were squishier than all hell, so viably if you _were_ stunlocked during your ER ragefest, you _would_ actually die.
Spear throws that idea out the window. Now it's a bruiser item mitigating all of a bruiser's weaknesses immediately on buy - it may be expensive, but you cover all your early game bases with the item: health for your early bulk, AD to do damage, and your cooldowns are effectively rendered irrelevant. It has the effective power of Triforce with less cost.
This item should never have existed and yet you're insistent on putting it in. Like, why the hell don't you realize that giving an item the ability to mitigate an entire class' weaknesses is completely unhealthy?