The benefits of tooling up for mass production only matter if you plan to make a lot of something. A cheap consumer computer case that sells for $63 requires millions and millions of dollars of tooling and equipment to produce them for that cost, and it requires that a large volume of them are sold to pay for that initial capital outlay.
If you only needed one computer case it would probably cost you many thousands of dollars (instead of $63) to make a single one as a bespoke item. The cost for one is tens or hundreds of times more than it could be, but if you only need one or two then the millions and millions you'd spend making a mass-produceable design make that one or two even more expensive yet. It's just not worth it.
A computer case is basically a box - its fundamental components are cheap and manufacturing costs dominate its pricetag. Modules for satellites are not cheap - they are filled with expensive and complex parts. Even removing the production costs, these are expensive components, so targeting the manufacturing costs, or especially the assembly costs, isn't necessarily going to provide the same gains, especially if the overall demand volume is weak.
Satellites are like this. Sure, we could make an assembly line to crank them out, but there's no point if you only even need two or ten or twenty of them. Exceptions exist, of course. SpaceX Starlink is a good example - there you have a heavy filled with dozens and dozens and dozens of mass produced, identical satellites. Naturally the production process for thousands of Starlink satellites is going to be much different from that of a single atmospheric probe destined for a highly specialized mission to Venus.
Space missions are usually highly specialized - everything about the satellite is optimized for its specific mission profile. How many batteries do you need? Are solar panels going to even be useful? etc? If you're going to the inner system it's much different from heading out to Jupiter - the way you power the satellite is different, the heat handling requirements are different, the amount and types of propellant will be different, the radiation shielding requirements will be different, the science you're trying to do will be different, etc.
All of these special challenges mean that it is usually worthwhile to customize the entire satellite for the specific mission, and no two missions have the same requirements. This greaty eats into the potential benefits you might find trying to recycle sub-components when so many parts need to be modified for this mission or that. Add to that the fact that every kg you can shave off of your design by removing generic things you don't need saves you almost $3k in launch costs alone (to say nothing of hauling that mass around for the entire mission), even today when launches are cheaper than ever.
There's very little that is generic about most space missions.