Because they don’t actually understand what religion is. They don’t understand what it is because they didn’t grow up with it, and are not predisposed for spirituality. If they did then they wouldn’t be so flippant.
Religion is not a ‘mere’ philosophical position. It is a deep, complicated constellation of values, morals, traditions, cultures. In the not-too-distant past, religion formed the core of every society. Belief was non-negotiable. Even if you found it silly, it was impossible to escape. You found yourself chanting right alongside the real believers, enjoying the sense of community.
It digs into your brain and, once it’s there, it never lets go. That experience has mostly disappeared in recent decades and can be hard to find. An atheist can sit in church and never feel that pull, because they were never exposed to it as a child. Only someone like that could consider belief in God to be the same as belief in unicorns.