A Ceramic/glass/metal IC housing can offer hermeticity (gas, water and sh*t proofness) satisfying aerospace/military/industrial standards, while plastics cannot do so completely - gases and liquids will slowly diffuse, especially with plastics used for IC packaging often not being a simple blob of resin but a compound material made of more sand than plastic.
Most plastic materials also can take less of a harsh temperature regimen than the semiconductor itself - even if it doesn't melt or burn, trapped water (not truly watertight, as mentioned above) can burst or embrittle the plastic if it turns to steam on sudden temperature changes.
Usually, ceramic packaged versions of components are considered nearly synonymous with military/industrial/aerospace grade parts, which create an impression of especially high quality and reliability (in some parts deservedly so, acceptance testing for these is far stricter).
Also, small/early production runs might have been semi-manually bonded into cases after production - ceramic case kits are sold for such purposes. Also, a lidded ceramic package can be easily taken apart again without destroying everything inside, which can be valuable for defect analysis. Also, you can easily use a semiconductor die or even a multi-die arrangement that is not protected suitably for embedding into plastic.
Finally: There is aesthetics, especially with the gold plating commonly found on ceramic parts. Given that 1980s era computers were expensive anyway, making key components look highest quality was probably considered worthwile. Even electrically nearly irrelevant metal parts were gold plated on chips meant to be blown up along with missiles :) - my theory is that it was about corrosion protection.