It's true that superb pixel art is harder to pull off than superb 3D models because of greater self-imposed limitations. However, it's also true that if you just want your art to be passable, it's easier to achieve that via low-res pixel art than via amateur 3D modeling. IOW, pixel art has both a lower barrier to entry and a higher skill cap.
Which is exactly my point: if we're resorting to external assets, there are infinitely more spritesheets available online for the low, low price of free, because making decent sprites is such a comparatively small effort that even artists themselves attach less value to the task.
(Which still isn't to devalue the effort that it takes to make good-looking sprites; I've tried, and damn do I suck at it.)
You need a baseline of art, and then you need to tweak the animations to match your video game. A model may not have a "double-jump" animation but you can easily add one through bone manipulations with any 3d model.
But if a sprite-sheet were missing the double-jump animation? You're either doing it yourself (hard pixel art style) or hiring someone else to do it.
