Our blogger had this response:

Even though $1 billion is an arbitrary threshold, these specific events account for the majority (>80%) of the damage from all recorded U.S. weather and climate events (NCEI; Munich Re). In fact, these billion-dollar disaster events are becoming an increasingly larger percentage of the cumulative damage from the full distribution of weather-related events at all scales and loss levels.

We do have lots of data to analyze smaller events, but details are not always available or consistent on smaller events. Larger disasters are more likely to have complete info. 1980 is the beginning of the first decade in which most of the public and private sector disaster data resources we use become available. 

For context, the 42-year (1980-2021) annual average cost for these billion-dollar disasters (inflation-adjusted to 2021 dollars) is $51.4 billion. However, the total cost of U.S. billion-dollar disasters over the last 5 years (2017-2021) is $742.1 billion, with a 5-year annual cost average of $148.4 billion, both of which are new records and nearly triple the 42-year inflation adjusted annual average cost. 

Also, we do introduce events into the time series as they "inflate" their way above $1B in costs in today's dollars. Every year, this leads to the introduction of several new events added from earlier in the time series.