[reeckoh]

Good question - I'm even typing this with thumbs now!

Thinking back, it was during a time when most phones were small enough to fit comfortably in one small hand. People would hold it with one hand, and use the other to tap.

Modern phones are too heavy and bulky for that, so I think we use both hands to hold and use them. Or maybe the finger-pecking was unrelated to phone use, who knows.