Ok, then why not an abstract object?  Why would you prefer the hypothesis that God made the universe rather than an abstract object?

Well, the thing about abstract objects, like numbers, or formulas, is that they don't stand in "causal relation" to anything. Things like numbers or physical laws may exist, but they have no power to create anything or to make a new event happen, because those types of objects are passive. In other words, the numerical system or formulas, no matter how complex, can passively describe reality or describe a pattern of behavior in the universe, but they can't start a new event out of nothing. That would be like saying the number 42 can create a new event out of nothing.

But we do know of one non-physical entity that can create a brand new event without any causal chain and that's the mind. A mind is a non-physical entity, and it also has the intentionality to create a new event. That's not some watertight proof, but that's one reason why one might prefer the God-hypothesis rather than an abstract object.

If you want to find out more about God and the beginning of the universe, I want to encourage you to check out other topics such as the Kalam Cosmological argument or the Fine-Tuning argument.