Because things like THAT are part of their beliefs. "Oh, I don't believe that part" no, fuck you, you either follow the religion or don't.

I'm not meaning this as a sarcastic question, but have you studied the bible? I'm assuming your comment is particularly directed at Christians since that's the most common religion which seems to pick and choose.

Actual study of Christian theology shows there is a shift in gears in the coming of Christ. The old laws aren't just tossed aside like "Oh we don't believe that part". Instead, they are fulfilled by the death of Christ and by being that fulfillment he opens a new covenant.

The New Covenant revolves around love. By Christ's dying the wrath of God (and the motivation behind all the OT nastiness) is sated. Christ serves as an intercessor, like the ultimate defense attorney who says "Yes they're guilty, but I'll take their sentence myself Your Honor". This means all the old laws are wrapped up under one; Love the Lord your God with all heart, mind and soul, and love your neighbor as yourself.

Following that one law fulfills all the rest. So it's not that they don't "believe that part", it's just that part isn't relevant anymore.

You probably won't be willing to see the difference, but it is there. One reason it is so hard to see is the fact that the Church and Christianity have taken a really simple concept and mucked it all up. Rather intentionally at that.

This really only applies to Christianity, but as I said I assume it's one that you often think of when considering this. It's a bit relevant to Jews to, but they've decided to call "doesn't count" on the whole Jesus thing so that's that.