Should they be fast-tracking promising, unranked guys straight to the top, without them having to really prove themselves?
That wouldn't be a good deal for anyone really. Fans would quickly lose track of who is who, if new names would be popping up too often. And the UFC and the fighters wouldn't be earning as much as they do when there's a clash between the top fighters with established brands.
It's not like the top is outside of the reach of the new and prospective fighters, they'd still get their chance eventually. Look at Aliskerov, he's currently outside of the top 15, but he was given a chance to fight a former champ and a number 3 guy. If Aliskerov wins, he will be trampolined to the top.