The chance of a parked bike being stolen at a specific place and time depends on two factors:
Motivation of nearby people to steal bike - whether opportunistic or organised, the thief needs to have that thought in the first place ;
Ability of nearby people to steal bike - possession of relevant tools (more specialist tools needed to break better locks), speed of getaway, confidence to continue with theft if challenged, etc.
So for example, in central London there are 'full time' thieves with tools who are both motivated and able to steal bikes. The chance of theft of a nice bike is high without a decent lock.
In a rural village there are fewer people about and probably no professional thieves. You might get away with leaving a bike unlocked by the duck pond overnight.