A lot of Android phones that aren't made by Google will have a version of Android that looks and feels quite different than Android on a Pixel smartphone. That's because those non-Google smartphones are running extra layers of software and user interfaces on top of Android to give their phones more features. It also allows them to give Android their own design that companies might think looks better on their hardware.
Samsung is one of those phone makers that alters Android to give it more features and a different look. Samsung's user interface has improved over the years, but nothing beats the real thing. Yes, Samsung phones have more features like iris and face detection (among several other features), which are things that Samsung couldn't add to its phones if it stuck to normal, pure Android.
The thing is, I've been using Google's Pixel 3 XL for the last week or so, and I haven't missed any of Samsung's extra features in the Galaxy S9. Plus, I strongly believe that Google's pure version of Android doesn't need any design alterations. I also get the nagging feeling that the extra stuff that's floating on top of Android negatively affects the Galaxy phone's performance.
To boot, Pixel phones also get the latest Android updates directly from Google. It doesn't matter too much to smartphone users, but it's still good for us and the Android ecosystem if more phones are running on the latest versions of Android.