Tmazz has presented very good points. Thanks T.
The design is also different from the standpoint that a preamp is designed for itself sake, naturalness, while in an integrated, the designing process is typically output and then towards the front/first stages.
As a consequence, the preamplifier stage in an integrated amp is designed to correct for the problems of the later stages. The more accurate/natural the later stages, the more accurate/natural the first/preamplifier stage.
Conversely, the less accurate the later stages, the less accurate the preamplifier/first stage.
One has to also consider the speaker and room when designing the totality of the integrated. The preamplifier is typically designed, at least for me, on an absolute basis. This means I design the speaker/amp as a unit, also considering my venue acoustics.
Cheers
Steve