Because it’s not just desktop search in the conventional sense, it parses and indexes EVERYTHING that is shown on screen regardless of app.
Normal desktop search is fine for ‘where is file X?’ or ‘which document contains Y?’ but the point of Recall is to be able to deal with far more wide reaching queries, like ‘what website did I order a cake from last week?’ or ‘where did I see a great idea for a pizza recently?’, where it can find sources for natural-language queries across anything that was shown onscreen, whether it was a website, an email, a slack message, or something within any other app.
As always, the headline should be qualified with ‘… in the USA.’