The following may not be entirely accurate to the last detail, but it illustrates some of the history well enough:
Slider is just the MDC version of SeekBar - named a little differently and with a lot more functionality.
First, there was only SeekBar - the original framework UI component bundled with Android.
Then, AndroidX libraries, previously called Support Libraries, were created for backporting new UI components and bugfixes to older phones which OEMs refused to update anymore. The AndroidX AppCompat subproject swaps out all your framework components for the "fixed" versions whenever you use AppCompatActivity. SeekBar -> AppCompatSeekBar, TextView -> AppCompatTextView...

The old Support Libraries also had a package named design which contained some of the then-new Material 1.0 components (CardView and whatnot) which didn't have a framework counterpart. That's the reason why there is no AppCompatCardView - because there is no framework CardView, so there is nothing to "fix" with AppCompat.
If I recall correctly, the design package of the Support Libraries later became the starting point for the Android implementation of Material Components when Material 2.0 was unveiled. Material Design has its own version of SeekBar which is called Slider and it can do everything the old Android SeekBar can plus a lot more.

As for which one to use? (Neither is "better", that kind of thinking is inapplicable)
- If you're using the MDC library already, or if you need some of
Slider's advanced capabilities, use Slider.
- If you only need a simple line with a circle sliding between two extremes, use
SeekBar instead of pulling the MDC library just for that.