Why does this crystal oscillator have a window?

@SteveHubbard has it right. The crystal has to be hermetically sealed preventing gas migration into the chamber, so plastic alone will not work. Glass is an excellent material for creating gas-tight hermetic seals with through-conductors. The crystal chamber will be filled with a dry inert gas or a vacuum.

The tutorial from Epson digikey.com/en/ptm/e/epson/spxo-and-vcxo/tutorial implies that the crystal is encapsulated in a ceramic package and that the chip is either inside this package or the package and chip are embedded in a plastic package. I am guessing that the lowest profile is obtained when one side of the ceramic package is exposed and that this glass is part of the ceramic package. Perhaps it isn't practical to encapsulate a crystal in plastic alone?