100 hours is a nice round figure and is based on nothing more than an assumption that by the time you have 100 hours total time, you probably have sufficient experience to cope with a particular aeroplane or airfield. It will not apply to everyone, but its probably a good statistical average. Many pilots will have given up by 100 hours whilst some will only just have qualified; others may be highly proficient.
Groups don't like having their aircraft grounded through damage, strip owners don't want aeroplanes through the hedge so they have to set a minimum experience level. A number of insurance companies also place requirements and I am sure they have statistics to indicate a minimum level. As has already been said you can have 1000 hours of experience or you can have 1 hour 1000 times, more frequently referred to as hour building!