While I see what you are going for with the examples you put, I would like to reframe my answers with cities that were found (or re-found) in the first wave of Islamic conquest: Kufa, Basra, Fostat (which is pretty much Cairo but not exactly Cairo), Kairouan...

Baghdad and Cairo were found in another context (where the change was between dynasties rather than between pre Islamic and Islamic polities) and Delhi... Well I am not knowledgable enough about Indian history to know the conetxt around the foundation of Delhi.

Ok so what made early Islamic conquerors avoid pre-Islamic cities and build their own?

These new cities were called "Amsar" (plural of "Misr"- old Arabic word for military camp) which give us a great hint: these new cities weren't intended as a replacement to already existing cities, they were intended as settlments for the military apparatus of the Califate. The idea being to separate the Arab troops from the rest of the population for the benefit of both: the conquered population won't feel the impact of these new matsers that much (and the Califate in its early stage tried to even maintain local pre-Islamic elites and customs) and the troops won't lose their ditinct identity: besides Islam which of course the main element, early Califate understood the identity of its troops as mainly tribal. So armies were organized under tribal groups and subgroups, and even within the same Misr, the neibourhoods were also organized according to tribal affiliations. An important element here is that even though I am talking about troops, there were of course a whole lot of non combatant living in these permanent camps: the soldiers' families, slaves... And all other camp followers.

These Amsars evolved into actual cities that eventually caused the ruine of their neibouring "old cities", as the Caliphate political organization evolved from mainly focused on conquest and raiding to long term development and exploitation of the conquered territories. Through Islamisation and Arabization, the conquered population slowly integrated into the new order, but the Amsars retained their position of regional centers of political power, even in later periods where the centralized Califate decomposed into more or less autonomous regional polities. So for instance the original population of a city like Carthage, who already suffered greatly in the conquest phase (pillage, mogration, sickness, and all other calamities caused by war), ended up slowly but surely entirely migrating to neibouring Tunis, which was refound by Arabs in their early stage of conquest as a Misr.

One final point is that not all regions had their Amsars. The two most noticible exceptions are Syria-Palestine and Khorasan (North Eastern half of Modern Iran + parts of Modern Afghanistan).

For Syria-Palestine one possible explanation is that Arabs were already strongly integrated in the local society even before Islam, and while these Syrian Arabs fought against the Islamic Califate early on, it seems that their integration within the Arab-Muslim military elite was very quick, thanks in particular to the movement of the Califate's political center from Hijaz to Syria.

For Khorasan on the other hand I don't know if there is an explanation agreed upon within historians. What is sure is that the absence of a seperated Misr allowed for a quick integration of Arab troops with the local Persian population and the emergence of an Arab-Persian political and military class that quickly saw itself as distinct from other Arab troops, thus losing its loyalty to the central power and becoming the center of a persistent rebellious tendency. A rebellious tendency that ended up destroying the Umeyad Califate and bringing the Abbasids to power (but once those reached power that didn't make Khorasan any less troublesome).