I see this everywhere here

Because that's the default setting and if you submit many memes at all it's just one more hassle to always change the font before composing your text.

truth.

I thought the default setting was white

Sorry, I thought you were talking about the font itself. Yes you are correct, the color variations are personal preferences. In my case I do so because I long ago got tired of plain white. In my case it's just that simple. As for other memers, I am sure that each has his or her own reason.