Grateful yet…
Case in point: other people setting up a birthday surprise for you with a cake and attending friends/family.
Why would anyone say, “Thank you for the surprise. Simple yet meaningful.”?
It’s definitely not simple because someone bothers to get out of the way to buy you a cake and remembering you and making sure that you know everybody remembers you.
That’s not simple at all.
You could have said, “Thank you so much!” and you can further add (if you mean it), “I love it!” or “I’m so happy!” or “You’re the best!”
But…
Then why does the saying exist? One would ask…
The person prepping the surprise could say that to mean, “I hope you like it.”