Yes, some people failed to read what they were buying.
Also, whilst there was clearly a problem with misselling, exacerbated by commission, I wonder whether some of those claiming they never knew they had PPI did in fact know at the time. For example it was quite well known that those with poor credit ratings would not get a loan at a decent rate without it, and some were prepared to pay it as an insurance policy but as they never needed to claim on the insurance it is convenient now to say "I never wanted it/knew I had it".