Other reasons include:

-the amount of tools at disposal that's meant for creating the stuff in the 1st place. For the creature & vehicles, a majority of them have abilities usable in stages they're prominent at (& the adventures from the GA expansion pack)

-an awesome as hell expansion pack called Galactic Adventures, which allows to create adventures & give your creature some sick-ass abilities with the captain outfiter

-the music sounding heavenly. Damn, that menu music is undescribably stunning & magnificent

-the way stages function here is executed well (to me, at least, but it's fun, so it doesn't matter). My favorite stage is the creature stage (ofc), but other stages have their merit, especially civilization & cell

-its online functionality. Spore has a community that makes some great creations, which can appear in your game if downloaded

There's likely some other reasons I could've put here, but these ones are what I could remember for this post. All in all, Spore's a fun time that's more than just fun... now if only I could play it with the expansion without the expansion not working :(