The quick answer is that glued in necks represent the traditional way of building an instrument and can be seen as a throwback from the violin building industry, which dates back a few hundred years.
Bolt-on necks represent a relatively new building tradition, which was popularised by Leo Fender in the 1950s. Leo was an engineer and he considered that having replaceable necks meant fewer spoiled instruments as defective necks could be very quickly replaced, whereas glued-in necks which were found to be faulty after manufacture often weren’t.
Sound wise, guitar folklore insists that glued-in necks offer greater sustain and bolt-on more dynamics and ‘twang’.
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