I don’t know. It’s an assumption, backed up by not having encountered any evidence to the contrary. It seems the most reasonable explanation for the universe we see.
It could be that there is no objective reality. If so, none of my conjecture or belief makes any difference - it would be impossible to know anything.
Do you not believe in an objective reality that exists outside (at least human’s) our minds ability to perceive it?
Only if there is objective truth. Otherwise, everything is subjective.
To follow up: you expect me to believe in an objective reality outside of my ability to perceive it, yet you reject God because He is outside your ability to perceive Him.