Here's a fun answer: https://tvtropes.org/pmwiki/pmwiki.php/JustForFun/HowToInvad...
Just the "Motivation and Planning" category will reveal several reasons. My favorite is that there is almost no reason to go through the trouble of colonizing our planet, if you need any of our resources you can mine asteroids and no one will fight back. Unless of course the aliens are Evil.