To realise why vendors do this, you need to understand the Psychology behind it:
Firstly, many properties have a 'catch' e.g. no parking, small garden, road noise etc. etc. The vendor will have spent years learning to live with these and they won't appreciate how offputting they are or why they may lower the value to any potential buyer.
Secondly, there is usually a strong emotional attachment to property - the vendor's home - and to lower the price means that essentially the vendor is devaluing something that means an awful lot more to them than to anyone else.
Thirdly, many vendors will be adamant their home is 'special' in some way and when they read articles about prime property/good areas holding up they automatically assume this applies to them.
Finally and most importantly, many vendors simply don't grasp the economics behind the housing market. They believe that the 'boom' years leading up to 2007 are the norm and that the current market is a 'blip' and will recover.
They don't realise that the lending conditions that pushed prices up (e.g. 10x income mortgages, 125% LTV mortgages, self-certification, interest-only mortgages etc.) almost collapsed the entire western financial system. These conditions will not return for generations, if ever. It sounds crazy but now most buyers can only borrow what they can actually afford to repay (and btw why do you think lenders now require an average of 20-25% deposit - they're greedy and immoral but not entirely stupid).
Even those few areas which are currently holding up (e.g. central London) are doing so mainly through investor money. The problem with this is that these markets are prone to the same whims as other investment markets i.e. can fall rapidly if investors see a better opportunities elsewhere and/or sentiment turns.
And of course all of this is not helped by Estate Agents overvaluing to win instructions or newspapers trying to sell papers by spinning the figures to give the impression that prices are 'soaring'.
Unfortunately, I suspect many of these vendors will still be waiting for the recovery in a decade's time.