ثُمَّ لَمَّا كَانَ مَبْنَى عِلْمِ الْكَلَامِ عَلَى الِاسْتِدْلَالِ بِوُجُودِ الْمُحْدَثَاتِ
Because kalām is founded upon reasoning from the existence of originated things.
The term muḥdathāt concerns things considered as having originated. Their existence, changes and conditions provide material for reasoning about dependence and the Creator.
Two steps must be kept separate: acknowledging that things exist and then establishing further premises about their origination and dependence. An observation alone does not automatically establish every step in the argument.