They get compared because Starfield borrows a ton of gameplay concepts from NMS without even adapting them much. The exploration gameplay is quite literally identical to NMS except with uglier procgen planets, fewer abilites and no freedom to fly around everywhere.

Which is why people who were interested in the exploration aspect tend to be more disappointed with Starfield. The rest of the game is typical Bethesda stuff which is fine, but exploration outside of cities gets really stale really fast. I think that disappoints a lot of people, including me, because exploration was one of my favorite parts of previous Bethesda games like Skyrim