Because a lot of new-build houses are simply not constructed to acceptable standards - mainly to save costs.
We moved from a 1937 house to a 1977 house, and the difference is noticable - moslty the walls, which were nice thick brick in our old house, and the internal walls seem to be plasterboard and studding in our new one.
I don;t think I would even consider a brand-new house - the shoddy finish on even the show houses we saw when we were searching was so bad - lord knows what the average house on the estate would be like if this is the 'best' they can show prospective buyers.
If it's your house, have a word with your construction company and get them round to finish the job properly.