Different languages, different grammar.
In English, a year typically does not stand alone. Most notably, if somehing happened in a year, this is rendered by using the preposition in. An alternative is to say ‘in the year …’ or others.
David Bowie died in 2016.
It was in the year 2016 that David Bowie died.
French does something entirely analogous using the preposition en.
David Bowie est mort en 2016.
C’était en l’an 2016 que David Bowie est mort.
Finnish only really uses a construction along the lines of being the year for similar sentences:
David Bowie kuoli vuonna 2016.
And a similar version is known for Latin where the construction is — at least, according to my experience — anno MMXVI. No example sentence because I do not speak Latin.
German, on the other hand, does not need the qualifier in a year except for certain situations in which misunderstandings may occur (although they typically still wouldn’t) — such as years lower than 30. Instead, the year is simply put into the sentence as is, no preposition needed. An extended form of ‘in the year …’ still exists, however.
David Bowie starb 2016.
David Bowie starb im Jahr(e) 2016.
(The dative-e is optional but often heard because using im Jahre gives something a gravitational, historical sound.)
This is just the way German grammar works. For comparison, here is a German translation of the second paragraph just to show how similar the languages work in the background with the exception of the lack of a preposition. The bolded part is identical — note how it uses a preposition and the dative case as expected.
Auf Englisch steht eine Jahreszahl typischerweise nicht alleine. Insbesondere wird, wenn etwas in einem Jahr geschah, dies durch die Präposition in ausgedrückt. Alternativ kann man ‘in the year …’ oder ähnliches sagen.