why is a 'polishing cloth' not included in the package?

I'm not sure of Apple's official reason, but I suspect that it's because there's almost no point in cleaning it, given that using it necessitates touching it and thus constantly causing prints. Also, the surface is actually very tough compared to other various surfaces, or a pair of glasses, for example, so it's not necessary to use an especially soft material. I clean my off constantly with regular towels, or even by rubbing it on my shirt sleeve, and I've never had an issue with it getting scratched.