The garage is not counted in the square footage of a house  because the garage is not considered to be a finished living space. By code in most places, only finished living spaces such as the bedroom, living room, etc are considered as living space because they have a ceiling, wall, roof, accessible by the door without leaving the house, and heat and cooling source. Since the garage does have any of these features, it is not considered as a living space thereby not counted in the square footage of a house for an appraisal. 
In the United States the garage is not counted in the square footage of a house , because the garage is not considered to be a finished living space. By code in most places, only finished living spaces such as the bedroom, living room, etc are considered as living space because they have a ceiling, wall, roof, accessible by the door without leaving the house, and heat and cooling source. Since the garage does have any of these features, it is not considered as a living space thereby not counted in the square footage of a house for an appraisal. However in other countries, such as New Zealand, internally accessed garages are considered as constituting a direct extension of the house and accordingly are quoted as part of the total size."Should garages count as the home?", New Zealand Herald 23 December 2017
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However in other countries, such as New Zealand, internally accessed garages are considered as constituting a direct extension of the house and accordingly are quoted as part of the total size."Should garages count as the home?", New Zealand Herald 23 December 2017