Brazen theft but it's been used in Duke Nukem terribly, the game looks awful compared to Turrican. I suppose it looks like what it is, a bunch of stolen graphics cobbled together by a poor artist.
I don't know why it's so popular to bash games while you know nothing...
First of all the whole duke nukem games contain far FAR more tiles than those. Secondly most screenshots are from Duke Nukem 1, which was running in EGA. EGA means sixteen predefined colors
. Compare this with the 4096 colors of Amiga and you can see why the graphics look so poor.
As to why the artist copied graphics? Many reasons... but we don't know them, we can only speculate (although to be honest, i'm almost sure that even if the reason was known from the artist himself, nobody would listen to him and decide their own "reason" instead).
In any case, note that the tiles there is not copied
, but replicated and it's not identical either. Most tiles are modified in one way or another.