Because when you take your five year old out onto the cul-de-sac to give them cycling lessons, you'd have to buy insurance for it. It's the floodgates argument.
That's simply not true. Horses walking and tractors are generally slower. Do you drive up to their arses beeping and overtaking 2 feet from them?
Treat cyclists on the road like you'd treat horses on the road and we'll all get along just fine.
We don't do that to tractors because they don't drive two abreast on a dual-carraigeway having a conversation, they have a ligitimate reason for going slow AND taking up space, a cyclist can do something about it and ride single file so as not to hold up other road users.
This is why cyclists have so many problems, expect to the treated like gods gift but do nothing to help themselves when given the opportunity.