It’s worth mentioning that they often do this to reduce flash size, i.e. save 💰. Virtual AB was introduced to help with this but it’s relatively new. With it, there’s no need to reserve the space for 2x system partition. The needed extra space during update is taken from /data and released post success. There’s also a compressed virtual AB scheme now which helps reduce the space needed from /data to enable really space-crippled devices like CCwGTV and other Android TV things like Sony and other TVs.