Because our society expects that a high school diploma means something other than a child managed to show up a bare minimum number of days for 13 years. Employers, Colleges, and Universities expect that a high school diploma means someone has achieved at least a minimum level of proficiency in basic areas.
Detroit Public Schools has or had mandatory advancement. If you showed up you moved to the next grade regardless of grades. Do that for 13 years and you received a diploma. The state of Michigan responded by putting in place a standardized test called the MEAP. If a student didn't pass the MEAP they didn't get a state certification on their diploma. In more recent years similar to NCLB, there have been attempts to tie funding to success rates on the MEAP as well. This was done in part because there was nothing in place actually holding local school districts accountable for the tax dollars they were receiving.
People want accountability because invest billions of dollars a year in educating children. Almost all education funding is done at a state level now. Going back 100+ years we've had standard curriculum expectations of public schools, usually set at at a state level or at minimum there were recommended state guidelines. You may pay local property taxes, but in most states that money actually goes to the state who in turn give the local school district a flat rate per child. Local communities usually aren't actually funding their own schools in reality anymore.