It has nothing to do with Thai customs. It has to with being willfully brainless.
If you speak fluent Thai and ask them and why they do that, you will stare at you with an utterly gormless, drooling look and say “I don’t know.”
If you press, they will get their manager and like ten people will become involved in answering a simple question.
Eventually if they finally understand they will be angry at you.
Most Thais aren’t actually stupid, but they are so averse to thinking they get angry if you ask them to. It’s offensive to them.