There's a slight difference though, Debian focuses on stability, and their end goal is to release a new stable branch every so often. As such, testing doesn't get security fixes quite as fast as stable, and sometimes things break and doesn't get fixed until they are fixed upstream in Sid (unstable).
Ubuntu on the other hand rolls a lot of the packages into a release, and then supports that release for a set period of time, with regular updates and security fixes.
So, even though Ubuntu stable is based on Debian's unstable, it's not as unstable as Debian's testing, since Ubuntu stable is a release, while Debian testing is just that, a testing repository for the next stable branch.
This all boils down to the mission of the different distributions. Debian arguably has created and keeps creating one of the most stable Linux distributions available, which makes it ideal for servers where you don't have as much software installed compared to a desktop machine, and you really care about everything not breaking. This comes at the cost of not getting the latest versions of packages, as it takes time to ensure the level of stability that Debian requires.
Ubuntu on the other hand has created a modern distribution that's more suited to Desktop environments, where you need the latest versions of packages to support more diverse hardware. Then they also have the LTS versions that are more suited for servers for the above reasons, more stability.
