That's not an ending. There must have been more
That's the question.
The final 12 verses as we have them are not in the oldest manuscripts we hold. In the 4th century, both Eusebius and Jerome stated that nearly all the known Greek manuscripts end the Gospel at v8.
There are also at least two endings. The common one is the short one, and both of them are stylistically and syntactically different to the rest of the Gospel. For these reasons scholars generally accept that the Gospel ended at v8. All modern Bible translations somehow indicate a break between v8 and v9-20.
If, as Hart suggests (and others agree) Mark was written to be performed more than to be read, then the abrupt ending serves a purpose – Don't go home, happy and satisfied with having heard a stirring tale, go
on. The last words of the tomb are to the disciples "he precedes you into Galilee, there you will see him" – the story doesn't end here, go where the story leads, now
your journey begins – it's a dynamic ending, and the whole text is a dynamic tale, written with the average person in mind.