The problem is that grid frequency has terrible mid-term performance. The frequency itself will be reasonably close to 50/60Hz, but it's guaranteed to be off by

The problem is that grid frequency has terrible mid-term performance. The frequency itself will be reasonably close to 50/60Hz, but it's guaranteed to be off by a little bit due to the way it operates - and that small offset accumulates over time. For example, the European supergrid only makes time corrections if it has drifted by more than 20 seconds. That's good enough for the clock on your microwave, but it's pretty bad compared to literally any other time source.