I mean ... there's a continuous scale of how much effort you spend to express gratitude. You could ask the same question of "well why did you say 'thanks' instead of 'thank you' [instead of 'thank you very much', instead of 'I am humbled by your generosity', instead of some small favor done in return, instead of some large favor done in return]?"

You could also make the same criticism of e.g. an automated reply like "Thank you for your interest, we will reach out soon."

Not every thank you needs to be all-out. You can, of course, think more gratitude should have been expressed in any particular case, but there's nothing contradictory about capping it in any one instance.