Love2D uses openAL for audio, FreeType 2 for fonts, DevIL for image loading and Box2D for physics. It can also use image fonts. It uses luasocket for networking and has a compression API built in.

On top of that there are love2d specific libraries people have written to deal with 2D games like GUIs and tile libraries.

Then there is the ease of debugging, where you can use lua to have runtime access to the table of variables and can print them on screen if you need to, not to mention dynamically loading new update and draw and input functions.

This is all to say that just downloading SDL is not going to get anywhere close to what love2d has included.