This idea of sending birthday greetings by email, or phone, or text, or internet based personalised e-cards have only been around since the widespread availability and adoption of the internet. For most people, the default method for offering greetings and congratulations has been via the post and mail.
Most people would appreciate and value a card or letter through the mail more, seeing it as symbolic of a greater measure of thought and effort - love, even - than lines of text on your phone, or an email can offer....