Peace be on everyone.
Why do people deny or have various doubts about God?
Edit / add: and what is the source of morality in life, of above mentioned people?
Possibly doubt arises because with all the religions populating a world populated by over 7 billion people who in majority believe in an invisible spirit that empathizes with the human condition, the world is what it is at the present moment.
Thereby leading some to wonder how omni-benevolence reciprocates the favor of worship and respect for the power of God. When it could appear to those same people that God doesn't return the favor.
As to the source of morality, religion has never imparted a morality in those devoid of same. If it did as an absolute we wouldn't have a religious history, most any religion, that records evil vile behaviors and deeds committed by those who professed their actions were executed in the name of faith.
Morality, at least in my view, is not universal. However, it is something that is rooted in the conscience and resonates to the right instruction afforded by family and demonstrated by culture.
I do think there are cases wherein if we were to humanize the particular deity of certain religions in reading the holy text that speak of that deity and their behaviors and instructions to their faithful and compared those to our unique model of what morality is defined as and by, we might realize that deity in many religions is no better than we. Just more powerful in the process.
This is what often gives rise to the observation that humans create their god's in their image and likeness.
And given the state of the world that may be correct.