And it is easy for a thief to see that a bag is heavy and slim to judge that it contains something valuable. Anf then the thief can just forcefully take the bag from you.

In Barcelona there are a lot of cases when a thieve on a motorbike would try to snatch a bag or women handbag from a pedestrian waiting on a red light.

With a back pack it does not work. The drawback of a backpack is that a thief can just open the backpack or even cut with a sharp knife and one would not notice. But more expensive back-packs with a good design have protection against cutting and easy opening while on the back and with a cheaper one can be held in front.