The problem with the @username syntax is that, while the functionality is as common as emojis, it depends on the implementation of whatever host -- so what you write on Mastodon will not be treated the same on Twitter, or Instagram, or Telegram, or wherever else you are @ing. So hashtags are a much more universally portable way of communicating, and don't require linkage to a particular account or service. Also, a pity it doesn't have a formal name, like "octothorpe". #HashtagsNotAtSign