It's not just the weight, there's also the complexity of handling 10 (or whatever) systems. It's not always easy to know when a system fails. How can you be sure that the output is invalid?

Detecting complete blackouts is fairly easy, after a while when a watchdog tickle is absent for too long, but bad output due to flipped memory? Consensus-style systems can be used, but they are also a complexity in itself, and the point here was to replace a hardened single point of failure with redundancy.