Their browser doesn’t say that it protects you from websites (including their own) tracking you.

Also, they consider it a problem if a website can detect if you’re using incognito mode: https://www.blog.google/outreach-initiatives/google-news-initiative/protecting-private-browsing-chrome/

Chrome will likewise work to remedy any other current or future means of Incognito Mode detection.

Having a signal sent to websites to tell them that you’re in Incognito mode would make things worse for users and would probably work about as well to reduce tracking as the Do Not Track header.