Why do men have to lower their gaze from hijabi women who also cover their faces? I mean, aren't they fully covered? What is there left to lower one's gaze from?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The fact that a woman is wearing the Niqaab or is fully covered, as you mentioned, does not mean that one is secured from being tempted by her. Despite that fact, she remains a woman and she may attract men or tempt them. The Prophet said: “The woman is an ‘Awrah, and when she goes out, the devil makes people look at her.” [At-Tirmithi, and Al-Albaani classified it as Saheeh (authentic)]

Al-Munaawi said in Faydh Al-Qadeer (when interpreting the above Hadeeth): “Meaning, he makes men look at her, to make her commit an evil or to make men commit an evil because of her, and therefore tempts one or both of them.” [End of quote]

In any case, merely looking at a woman while she is fully covered without contemplation, or scrutiny, or looking at her with lust or desire is not forbidden if there is nothing that leads to temptation, such as when the clothes are adorned or decorated in themselves. However, if there is anything that leads to temptation, or that looking at her is with lust or desire, then it becomes forbidden.

Al-Qurtubi said when interpreting the verse in Soorah An-Noor [Quran 24, verse 30] about the order to lower the gaze: “The sight is the greatest door leading to the heart, and it is the sense through which most evil reach the heart. It is for this reason that people fall into sin due to it [sight]; and thus it was necessary to warn against it. It is obligatory to lower the gaze from all that is forbidden and from everything that is feared to cause temptation.” [End of quote]

Moreover, Ibn Abu Shaybah reported in the Musannaf from ‘Aasim Al-Ahwal from Ash-Sha'bi, about the verse in which Allah Says (what means): {Tell the believing men to lower their gaze (from looking at forbidden things} [Quran 24:30]; ‘Assim asked ash-Sha’bi: “(What is wrong in) a man looking at a woman if he does not see anything that is forbidden (in her)? Ash-Sha’bi replied: “Why should you stare at her (to start with)?”

Sometimes, a man may look at a woman out of curiosity [without any evil thoughts], and then the devil takes advantage of that and leads him to commit something the consequences of which he will regret; a believer should always seek to be safe in matters that affect his religion.

Allah knows best.