*Because hugging is spontaneous, like laughing.
Whether you like it or not, handshaking & hugging are the norm, when you want to behave outside the norm the onus is on you to make that clear or avoid it.
I avoid hand shaking where possible, I don't expect others to ask if they can shake my hand*
Laughter is spontaneous and can be involuntary yes, but hugging is never involuntary. It’s a deliberate action that the hugger chooses to do. They may do it spontaneously but that doesn’t mean they can’t control it or decide not to do it.
Hugging is absolutely not the norm, unless you’re a teenage girl.
People don’t specifically ask if they can shake your hand because they don’t generally come up to you, grab your hand and shake it with you just bring a passive recipient of the action. They proffer their hand and it’s up to you whether you accept it or not. With hugging though, huggers invade your space, touch you, pull you into physical contact, squeeze you, inflict their touch and smell on you without ever checking if this is ok. In other contexts it would be deemed an assault.