ذَلِكَ بِأَنَّهُ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
This is because Allah has power over all things.
Allah does not obtain His power from something He intends to create. He was powerful before creation and remains powerful after it.
Power is a divine perfection; the existence of creation manifests its effects, not the origin of the attribute itself.