Why don't children's bikes come with safety harnesses attatched?

I understand that he is two, but would you rather him fall off the bike on his own, or fall off the bike and the bike goes down with him, and possibly injures him even worse? Because thats what will happen if you attach your son to the bike... most bikes for children under 5 years old come with stabiliser wheels anyhow so it makes it harder for them to fall. I don't think there is a gap in the market, he would hurt himself more if the bike landed on him...