A NASA report, looking at the US confidential reporting system ASRS, found that flapless take-offs (unintended ones) happen more frequently than we realise, but because the crews fortunately get away with it on most occasions, we don't get to know. There's a summary at
Distractions frequently cause flapless take-offs, NASA reveals
As for the reason, NASA says it's because the pre-take-off period is so unstructured and full of distractions.