Why can’t Christians agree on things?

So many times I see Christians arguing online over everything, some saying “this, this, and this is sin” and others saying it isn’t.

Here’s a little info that I can use as examples:

I listen to hard rock and heavy metal music and I think it’s not sinful to do so, but others will tell me it’s sinful

I think Christians can drink, smoke and masturbate all in moderation and it’s perfectly acceptable, others will tell me it’s sinful

I think Christians can marry non Christians, but should be cautious of who that non Christian is, I’m in a relationship with an agnostic girl myself, but others again will tell me it’s sinful

I think Christians can get tattoos and piercings, in fact I want both of those myself, but again I’ll be told it’s sinful

I know a lot of this falls under personal conviction. But come on people, aren’t we all God’s children? Why can’t we get along and bring glory to his name?

Just FYi: I’m a straight 21M if anyone was curious.

If I offended anybody I deeply apologize! God bless! Your honesty is appreciated!