Probably because he was asked to and paid a substantial amount of money to do it. Also, to suggest that Lewis wrote his own biography is about as sensible as believing Katie Price writes her own novels. I've not read Lewis's biog but I guarantee there are probably worse F1 biogs and definitely worse sporting ones.
Here, I must confess to having an unusual and perhaps rare problem which, I suspect, may be my one and only genuine allergy. Biographies make me palpably sick. So much so that if I touch a biography in a bookshop I have to depart immediately and reach for open air and a tissue. It is a most bizarre condition, to say the least and I can honestly say that I do not recall ever reading more than a smidgin of a biog in a book. Interestingly, though, I have managed to read extracts in newspapers and magazines without such a reaction.
I must check out the A to Z of Allergies to see if the condition is an officially recognised one although I do not feel the need for a cure.
Non-sufferers of siffert_fan's allergy have been found with houses full of double glazed windows, sofas and Apple products; sitting huddled up on one of said sofas watching the Piers Morgan show having donated the remnants of their savings to spurious charities, thus leaving their Icelandic bank accounts empty.
This doesn't really belong in this thread, as it is not about a driver, nor is it an autobiography, but an excellent book about F1 is "Rob Walker" by Michael Cooper-Evans. It is long out of print but gives great insight into the way it used to be. For instance: The Walker race team consisted of: Rob, his wife, a secretary, and two mechanics! And with that, he won 9 races and at times entered two cars! Of course, the wins were made easier by having Stirling Moss as his driver.