All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The wisdom in this is so that the graves remain outside of the Prophet's Mosque, due to the prohibition of the Prophet, sallallaahu ‘alayhi wa sallam, against taking graves as mosques. The three noble graves, while they are currently attached to the Mosque, are disconnected from it by the three walls around them. The Prophet, sallallaahu ‘alayhi wa sallam, and his two Companions were buried in the quarter of ‘Aa’ishah and not in the mosque. This is continually realized by having these walls. If these walls were removed and a glass barrier was installed, as mentioned by the questioner, this would place those graves inside the mosque.
When the Prophet's Mosque was being expanded, ‘Umar ibn Al-Khattaab and ‘Uthmaan ibn ‘Affaan observed restricting its expansion in the direction of the houses of the Prophet, sallallaahu ‘alayhi wa sallam, which included his noble grave. The final expansion also did not include the left side of the grave, so as not to put the noble graves in the middle of the Prophet's Mosque. As such, they are attached to the mosque but are not inside it. Further, the manners of visiting the grave of the Prophet, sallallaahu ‘alayhi wa sallam, should be recognized.
Allaah Knows best.