Because I couldn't think of a way to correctly determine the absolute path to rspec-core's executable and we are referencing it at https://github.com/rspec/rspec-autotest/blob/96411a075a53fdc49e136e05fd6be478eaa6417f/lib/rspec/autotest.rb#L13 and then using it to build the rspec command at https://github.com/rspec/rspec-autotest/blob/96411a075a53fdc49e136e05fd6be478eaa6417f/lib/rspec/autotest.rb#L53