Because the opinions of local people are disregarded when planning applications are submitted for "affordable housing" projects. The developers and housing associations work with, and offer sweetners to, parish councils and local authorities before plans are submitted for public comment. When local residents, the people who will be most impacted by the new development, try to provide feedback at so-called public consultations, they are invariably told that the parish council and the local authority are supporting the proposals. But parish councils and local authorities have given their approval to the developers and housing associations without having first consulted the local residents. Also since the concept of "affordable housing" is not well defined, it's too easy for developers to use sentimental arguments, rather than logical reasoning, to back up their assertion that the housing is needed. Also, housing associations keep telling local residents that they have "charitable status" which leads many people to believe that they are charities and therefore must be okay. But they are not charities, instead they are odd organisations that use their "charitable status" to avoid paying taxes, to get government funding (i.e. taxpayers money), and to hide their accounts from public view.
Currently in rural areas, planning restrictionscan be waived for applications for "affordable housing" if the developers and the housing association agree that there is a need for the housing and houses can be built on green field sites, in Areas of Outstanding Natural Beauty, outside village boundaries, and so on. However, too many of these developments once they've been built are being sold to, or rented out to, people from outside the local area because there really wasn't a local need for this "affordable housing." Planning restrictions should apply equally across the board to all proposed developments in rural areas to help preserve the countryside, to prevent much needed farm land from being built on, to protect biodiversity, to reduce the amount of out-commuting, to help deal with climate change, and to prevent small villages from becoming suburban sprawls. Also the process for determining local need for "affording housing" needs to be transparent and overseen by a truly independent body (one that's not influenced by developers and housing associations).