It is absolutely true that software can be finished, it's just that software appears to be dead if it hasn't had any work done on it for years. You don't need to keep adding features and changing the software ad infinitum.

Just like with your building analogy and with other car analogies presented here, software does need some maintanence every now and again to keep it up to date - with security fixes, compiling to a newer platform, integrating fixes from dependencies, etc. And yes while buildings may be finished they stil require regular maintance if they are used.