Debian by default has had ancient versions of stuff, and if you want third party software, Ubuntu has more mindshare so it was more likely to be tested - anything from graphics card drivers to Steam games.

It hasn't. Debian has releases every 2 years, same as Ubuntu LTS, and packages are up to date at the point of release.

and of all those patches, the ones that have staying power get upstreamed as Debian packages all over again.