1. It doesn't take long
Writing a thank you note doesn't take too much time. All you need is few high quality sentences to get the job done!
2. It can make someone's day
Don't you love getting cards in the mail?
3. It's personal
Talking about the gift you received and how you are thankful for it tells the person who sent you the gift that the time and money it took was worth it.
4. It's good for your character
It is always good for your character to express gratitude for things given to you.
5. It shows that you care
I know it would be much easier to send a "Thank you!" text or email. But, if you sit down and write them out, it shows that you aren't selfish and care about the person who gave you the gift.