Because people can be assholes
In the larger Dutch cities it is generally accepted that a bike should have at least two different kinds of locks, and be locked to something solid. Some people say that the locks should cost about 50% (or 10% according to lock manufacturer Trelock) of what the bicycle is worth. Personally I think that is a bit much, but there is some truth to it as the majority of bikes are sold either second hand or at a convenience store.
When it comes to locks it is inescapable that their strength is directly related to their weight. The challenge is finding the right balance. However, the unfortunate truth is that with a big enough bolt cutter you can cut trough most locks, regardless of their security rating in seconds, and for those locks that cannot be cut there is always the angle grinder that will open any lock within minutes.
Getting your bicycle stolen sucks. It happened to me once with a beater bicycle that I occasionally forgot to lock while it was parked outside on the street overnight, one day it was gone. It was bound to happen eventually.
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