No potential disrespect intended with this question: Have you ever fought in a combat situation against heavily equipped fighters, such as ISIS? I have.
As for Baghdad, they actually hold other cities, industrial complexes, oil wells and refineries, and other previously well secured areas that they were able to attack, seize, and hold (now) for years. The fact that they haven't taken Baghdad has more to do with ISIS being stretched, currently, with their resources spread across a broad area of both Syria and Iraq with ISIS being attacked on three fronts (Syria, Iraq, and "Kurdistan") but yet they are surviving and holding.
Do not underestimate ISIS. Those that do have done so at their own peril, as President Obama has experienced. Even Putin pulled his forces out after just a few months of direct conflict.
The alternative to your assertion could be "If ISIS was so weak, why have they not been overrun and defeated by the US Coalition, the Iraqi Army and Air Force, The Syrian Army, the Kurdish Peshmerga, the Kurdish PKK, the Russian Army and Air Force, and everyone else that is aligned and fighting against them?" For a rag-tag group of unprofessional, weak idiots... they've done pretty well keeping just about the entire world from defeating them.