MTBF is not an estimate of how long a single hard drive will run continuously before dying. Instead, it is a population-wide probability calculation.
To calculate a 1,000,000-hour MTBF, a manufacturer might run a test fleet of 1,000 drives for 1,000 hours (roughly 42 days). If 1 drive fails during that window across the combined total of 1,000,000 operating hours, the drive model gets a "1,000,000-hour MTBF" rating.
An MTBF of 1,000,000 hours means that if you run 1,000 identical drives 24/7, you can statistically expect roughly 1 drive to fail every 1,000 hours across the whole array.