Who knows.
Back in the day it was very easy to pile up on miles as an EXP. 100% bonus, cabin class bonus, aadvantage promotions (fly to JFK in January-February and earn thousand miles per trip, etc) credit card miles, etc.
I have had more than a million miles sitting in my account at one point.
By year end, I was either exhausted of traveling or I was willing to pay cash for personal trips to be able to re-qualify for EXP.
As a young single professional (no family) I really had no use for the miles.

Video conferencing, PDFs, clouds, etc. made my work travel decrease significantly and only then I started to redeem my miles. (I also got married, started visiting aging parents more frequently, etc).
I am left with 3,000 miles. LOL. It's all good.

Maybe that person still has miles from back in the glory days and is unable to burn them for whatever reason.

Also, you will be surprised of people's travel patterns. Some people live on planes, visit 2 continents per week, commute weekly 5000+ miles, etc.
You end up sitting on a pile of miles.