In the first place, we wouldn't say them unless we mean them, yes? But there are many instances where we say them and yet, it's due to our tongue being so used to uttering them. You can say it comes naturally. Convenient, more like. After a while, the intended effect of extending a grateful "thank you" diminishes, much like "I'm sorry" is meant to convey our deepest and most sincere regrets.

It's also hard to say these words verbally or face-to-face. It seems much easier to pen or text them down, preferring for ourselves to remain faceless for fear of giving away our true feelings. How ironic to hide oneself, when the idea of muttering "thank you" or "I'm sorry" is to come forth and portray our honest gratification or sincere apologies.

Ironic, indeed.