Because with historical (or is that hysterical ) rises as justification, people are prepared to risk what they perceive is a short-term unaffordability pain to achieve a long-term affordability gain. It all goes topsy turvy if those expected capital gains don't come through as quickly, and that seems to be unfolding now.
Respectfully I have to disagree. A large part of the property market is people who want to own their own for two reasons:
1) Over time, the cost of owning (ie interest, etc) reduces in real terms, due to inflation
2) Once you own your home outright, the costs of providing your home drop significantly.
Sure, there are investors who want to buy a property for capital gain, but do you think it is these people paying top dollar? Perhaps these people are the ones who search for bargains (I know I do, as do most here).
Many people do not buy because of potential capital gain. Also, just because your property rises in capital value doesn't mean it's more or less affordable to you. As above, the cost of ownership decreases with time, and this is not correlated to capital value.
Prices have risen significantly over the past decade for several reasons:
1) Easier credit, especially if you bring something to the table (ie good income, existing assets, etc)
2) Dwelling sizes have increased by about 50% over the past 30 years
3) Cost of building (development costs, building costs) has dragged existing stock prices upwards
4) People have higher incomes in real terms
5) etc - there are others
What this boils down to is that yes, housing costs more than it used to, but people are still paying. At this point in the cycle (very generally, location specifics vary), we are past the peak to the point where prices soften a bit (some more than others, some not at all). This is the property cycle at work. Sure, the last one was a big one, but it still fits what we would expect of the market. I would now expect many areas to stagnate for some time, as pressure builds for the next big surge, which I would see happening in 6-9 years time.