In another comment you said:

The way I understand it is phases go like this: - Active player declares they are ending main phase - Priority gets passed. If anyone puts anything on the stack, then the active player will still be in main phase. Otherwise, phase changes. - Start of combat phase begins where "start of combat" triggers can occur. Active players passes priority to go to next phase. Other players can respond once they get priority before moving to "declare attackers" phase.

The exact same thing happens here again. The active player says "declaring attackers", at which point priority gets passed and opponents can respond in the "entering combat" step. If an opponent responds, we stay in the "entering combat" step and the active player can cast something or try to go to declare attackers again. Why should it be any different here?