The memory, as little as it is, the Voyager spacecraft, must be of a different sort. Launched in the late 1970s, the electronics is still functioning, although with a few issues. That'll soon be four times longer that the Rover.
The Voyager craft were intended to operate for many years. The mars rovers weren't. The mars rovers also reside in a much harsher environment than the space probes which float weightlessly in a vacuum at a constant temperature.
There was no reason to design the flash memory to last much longer than the expected lifetimes of the wheel bearings or solar panels. Just because by some miracle those both lasted much longer than expected, it doesn't mean that additional investments of resources into the memory would have been justified.
That, I tell friends, is why I'm happy to drive a 30+ year old car. It has issues, but the hardware it's built from is inherently more long-lived than that in today's cars. A crank-up window just keeps working. One driven by an electric motor doesn't.
False. Cars from that era were routinely sent to the scrapyard when they were less than 10 years old because they were rusted beyond repair. Now the average age of US cars is over ten years, twice what it was in the 1960s. Old cars also required constant maintenance of problem-prone mechanical parts such as ignition points and carburetors.