It's because Zelda has a narrative to carry you forward. Apples and Oranges comparison.

You can have the same enemies and the same mechanics if there is more story and more depth.

Destiny is a looter shooter grinder MMO-Lite. You literally can only play the story a maximum of 3 times through. It is NOT a story driven game.

Therefore, having the same enemies rinsed and re-used with every major DLC makes it feel more and more shitty.

Compare and Contrast with other major MMO's, where new DLC often brings entirely new and unique races to the entire game ecosystem.

The skorn are just a reskinned fallen with a slightly new mechanic for some of them.

The taken before them were just a reskinned... everything.