Thessaly has the potential to be very productiveIn the end, any land gained peacefully in Thessaly is a net gain for Greece from OTL. Acquiring any amount of Greek land from the Turks will boost significantly the popularity of whichever party is in charge of the country.
In addition to that, the Turks have to take into account risk/gain analysis. In its current state, how likely is the Greek army to overrun Thessaly if it declared war ? And is Thessaly such a productive Eyalet that it is worth having a war over ?