Outside of cubesats and mass deployed sats like GPS and the European variant, almost every satellite is somewhat of a unique build. Even cubesats and GPS sats

Outside of cubesats and mass deployed sats like GPS and the European variant, almost every satellite is somewhat of a unique build. Even cubesats and GPS sats are special, you can't simply take a GPS sat and repurpose it for a TV station. The requirements can be vastly different (geostationary vs LEO, precise orientation vs simple attitude control). The same goes for space telescope. You can't just take a ST sat body, ram in a LEO weather satellites electronics and expect that to stay in orbit or work reasonably.