Because you spiked out your entire gem or Rails app without bothering to write a single spec. Then you wanted to see your test coverage to get rolling on it, so you install simplecov and it said you had 95%! But of course, that's a lie. It just has no idea you have so many vacant specs.
Just require this file in your Rakefile, like so:
require File.join(__dir__, 'tasks/spec_shame.rb')