In my experience, because they don’t ‘really’ believe. That, despite what knowledge or claims they might make to the contrary.
The majority of people take sincere belief or risk/death seriously.
If you threaten someone’s cherished possession or even life partner, you can certainly bet they will hop into action and pay up.
When you don’t see a risk, or think you can avoid serious consequence - you endulge because you feel you can or it is worth it to do so.
Hardly likely if you were under serious impression there was a likelihood of your damnation.
Aside, yes people make mistakes based upon pressure, fear and many other things. But those usually contain a modicum of balance and regret.
Those who frequently sin, often have adopted a ‘one more wont hurt’ strategy. Jesus is more their bar keep, counting a ‘tab’, than he is their God or Messiah.