is the reason because they may ask for more documents related to your presence in the country and you won’t be able to provide them if you have already left ?
It helps to understand that Thailand Elite is a club with a special visa as a perk. You don’t have to use the visa, but your club membership counts down regardless if you use the perks or not.
I flew from Kona in February of this year from Hawaii and Hawaiian airlines tried telling me I need the Thai pass thing from COVID times with the QR code that hasn’t existed for two years.