Why do you think God spoke through an angel to Muhammad, but He spoke directly to Moses? How about Jesus? Why in the Bible there is no mention that God was speaking to Jesus through angel Gabriel?
God spoke to Mohammad (s) at times directly to Mohammad (s) and some of Quran was revealed in that manner and (majority) of the time (most of Quran) through Gabriel (a). I believe that, because that is what Quran and hadiths state.
The wisdom is not simply about hearing words of God, but understanding them at the various levels of transmission to humanity. Gabriel shows how they can be understood by believers at various stages, because it his specialization with Angels under him, to extend God's special mercy and grace to believers till they are guided by Imam directly.
Also, aside from Gabriel, Michael also shows how to extend it to all of humanity (disbelievers included). So Michael and Gabriel were with him continuously, so that he can calculate his words to compliment the Quran in a very calculative way.
I'm not sure what Bahais teach about Gabriel and Michael, but Gabriel specials the title "Ar-Raheem", "ar-rahman" is Michael's delegation, and is the hardest of all roles of Angels.
The Quran is complimented by the Sunnah, but for Mohammad (s) to calculate his words to compliment them perfectly, Gabriel and Michael, show him various stages of how the words will be perceived by intellects, as they special in the unseen realm in the battle for minds and hearts.
Also, Ahlulbayt (a) calculate their words keeping in mind all this, but also the works of previous Imams (a). This why tafsir Hassan Al-Askari (a) is incredibly hard for hearts to accept and understand (some of it is fabricated but most of it is from Imam (a)), because it's one of the final gates build on top of gates, and unleashing one of the final knots on the tongue of Messengers (s).
Mohammad (s) tongue had a knot, and so couldn't say everything, because people ignorant propaganda, ignorance, and falsehood immersion.
Ali (a) was a gate to wisdom during his time, but, even his words were calculated in a way that couldn't unleash all wisdom.
The limits of their words as well as their penetrating ways, is all due, to what they perceive in the unseen but also what they learned from Gabriel (a) and Michael (a) in that regard. Their words are calculated keeping in mind all of humanity.