One of the reasons may be the passwords that few of us can manage safely.

At the root of the password problem is the cognitive phenomena called “interference of memory”, by which we cannot firmly remember more than 5 text passwords on average. What worries us is not the password, but the textual password. The textual memory is only a small part of what we remember. We could think of making use of the larger part of our memory that is less subject to interference of memory. More attention could be paid to the efforts of expanding the password system to include images, particularly KNOWN images, as well as conventional texts.

Most of the humans are thousands times better at dealing with image memories than text memories. The former dates back to hundreds of millions of years ago while the latter's history is less than a
fraction of it. I wonder what merits we have in confining ourselves in the narrow corridor of text memories when CPUs are fast enough, bandwidth broad enough, memory storage cheap enough, and cameras built in mobile devices.