My theory is that the people doing it are young enough to have never used a landline phone, so they were never "normalised" on how to hold one. Then they saw people on The Apprentice and similar shows holding it like a pie (because the call was on speaker for the rest of the group to hear), and assumed that's the correct way to do it.

Presumably either nobody's ever corrected them, or they consider the actual right way to be old fashioned/uncool, or they simply feel weird not doing it like that now that it's ingrained.

Whatever the reason, it winds me up a treat and I wish that I could tell them how stupid they look!