I can't remember the exact quote which i will try to find later, but Gandalf tells Pippin that the palantiri are beyond Sauron or Saruman's skill. But giving the fact both Sauron and Saruman are greater in power and are of the Valar Aule, surely they would be better craftsman?
It should be obvious that just being a Maia, or being a Vala, doesn't make one more powerful in every respect than all beings of a lower order.
There are other examples of this in Tolkien's writings; for example of the Maia Eönwë it is said:
Chief among the Maiar of Valinor whose names are remembered in the histories of the Elder Days are Ilmarë, the handmaid of Varda, and Eönwë, the banner-bearer and herald of Manwë, whose might in arms is surpassed by none in Arda.
Yet Eönwë is a "mere" Maia; but despite that he's still a mightier warrior than either Tulkas or Oromë.
It's well-established in the Silmarillion that Ilúvatar gave gifts to the Children that the Ainur do not have; for example it's said in the Ainulindale of Melkor:
But he desired rather to subdue to his will both Elves and Men, envying the gifts with which Ilúvatar promised to endow them...
It's crucial that this was before Melkor's fall, and that Melkor was the most powerful of the Ainur, but yet he was envious of these gifts.
Furthermore, and despite both Sauron and Saruman being in origin Maiar of Aule ("the maker"), Tolkien is quite clear (in Letter 131) that the function of the Valar is not the creation of new things (my emphasis, aside from the "not" which is in the original):
These latter are as we should say angelic powers, whose function is to exercise delegated authority in their spheres (of rule and government, not creation, making or re-making).
It should therefore be not surprising that Feanor was able to make things that a Vala or Maia could not.
Feanor was a force of nature. He was an exception among Elves and was said to surpass all the Children of Eru.
Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and in subtlety alike, of all the Children of Illuvatar. [Of the Sun and Moon and the hiding of Valinor]
The Valar did not even know the craft he used to make the Silmarils [Of the Silmarils and the Unrest of the Noldor]. Feanor may be nearer to Saruman in personal power than is supposed.