They did not volunteer telephone numbers as universal proof of ID. So their threat model was proportional to their intended purpose of the identifier. If bad guys steal your phone number and run up $100 of calls, the phone company would eat the charges and get the number back. Of course nobody did that because it wasn't worth it.
Imagine you own a medium-sized residential building, maybe 20 households live there. You issue them all with front door keys. You use pretty good locks, from a no-duplicate series that isn't trivially picked by amateurs. You figure that picking the door or forging a key would be pretty hard so there's no way it's worth it when someone could just kick it down.
Then, to your astonishment, a local jewellery store announces that anyone who has one of your front door keys can now store up to $1M of valuables in safes they've made accessible from the street. "It's totally safe" they assure your residents, "because how could anybody else get one of these front door keys? That'd be impossible".
Um. What? Before you know what's happening, one of your residents is trying to sue you for a million dollars because they proudly used a safe to store their $1M of Bitcoins for some crazy reason and (duh) somebody just got a duplicate key easily enough for way less than $1M and stole them.