The increase in overall capacity is not actually very large, and that’s why the capacity factor is effectively flat.

Capacity and capacity factor are not connected. You could have one wind turbine or a thousand and they would tend to have a similar capacity factor, all else being equal. If you need another illustration of this, consider this table of China's stats, which includes the capacity factor. It stays steady at around 20% in a time period where capacity has increased by a factor of over 300.