Housebuilders build what sells. You may not like it, but they (the successful ones at least) have built million pound businesses on knowing exactly what they can sell, for what price, to whom.
It is akin to asking, 'Why do shops sell such cheap crap clothing?' The answer is, that's what the vast majority of people want to buy.
Personally, I would not consider buying a property from a housebuilder built within the last two decades. look at it like buying a car. You're better to let someone else buy the 'new homes' and then buy it after they more out, and all settlement, quality, and maintenance issues have made themselves known.
Quality has dropped right across the board in house building. There isn't the money in the market to justify a decent return to an Architect, so most are design and build (read builder does what the F he likes), and the industry no longer employs professional QA procedures/personnel to inspect the build.
A far wiser investment would be either, buying a piece of land and managing your own build (you could save yourself about 30% of the retail value of a property by doing it this way, but you need to be looking at build costs of £150K+ to realise a decent saving. The economics just don't work on lower priced properties), or buy an old characterful building somewhere, Victorian, Georgian, Edwardian, whatever, somewhere where the community changes little, the amenities and transport are well established, and your property price isn't going to be affected in any meaningful way by volatility in the market.
I love Rightmove. It's a great site for window shopping homes.