Every part needs to last 3 months. If even one part breaks after 2 months the whole thing failed. Each part increases the odds that one will break after 2 months. The only way to work around this is to design each part to last for 5 years, that way the odds are high that all parts survive for the required 3 months. It also means that odds are reasonable the rover will work for much longer than 3 months, and in many cases you can operate in a degraded state for a even longer.