There's no requirement for the sitemap to be at /sitemap.xml, or even to be named "sitemap.xml". They may have one, but since the norm is to submit them directly to a search engine (thanks for nothing, Google), sitemaps aren't typically automatically discoverable.
It's not that ironic. Sitemaps are an "easy way for webmasters to inform search engines about pages on their sites." Hardly a requirement for a simple site.
Who said that sitemap.xml is always in that location? If it is not specified in robots.txt then they don't want you to know about it in case it exists.