Why do critics of Islam mostly ignore the core belief in Allah's Oneness?

As someone who reconnected with Islam (Alhamdulillah!), I often wonder: why do non-Muslims, especially Christian and atheist debaters, hardly ever touch on the most central part of our faith-the absolute, indivisible oneness of Allah, as emphasized in the Qur'an? They seem to focus on side issues or try to smear the character of the Prophet Muhammad (peace be upon him) and his companions, bringing up 7th-century social contexts without really understanding them. But they almost never challenge the fact that Allah is One, the Eternal Refuge, who neither begets nor is born, and there's nothing like Him. Isn't that the very heart of being a Muslim? So, why skip it?