It's not unusual. If you have a paper wallet or an address on an exchange or a custodial wallet, you won't change it for each transaction. It's not everybody who needs anonymity and it's more convenient to have only one address.
If you do not care about the security and privacy of your coins, theny why bother changing your address?
People are doing it because they wanted to attain an optimum privacy and security to their hard-earned money. This would be the very best practice if you are holding a large amount of cryptocurrency in your pocket. They just want to reduce the risk of getting tracked by people who wants to attack them virtually or physically.
There is nothing more convenient than knowing that you are not compromised.