Because we asked them to stop taking pictures in the bathroom.

I've spent years of my life trying to find the answer to this question (it's important!) And I'm still not convinced I know for sure, but until a greater revelation comes along... I assume it's a two part phenomenon. First part is that it produces a fairly flattering portrait because the harsh, direct overhead light is blocked by the roof, but strong, diffuse daylight is bounced in from all the side windows, including a bit from the back for some rim-lighting of the hair. This is actually somewhat similar to a professional studio portrait set-up, without the fuss and expense.
The second part is that, since it looks pretty good, other people see it and want a good look too, so it becomes a naturally self-replicating internet meme, without people having any conscious knowledge of why they are doing it. What works, gets repeated. In other words: evolution.

Big deal. I have one in my profile. IDGAF, lol! ♥

Same goes for the duck-face. WTH is with the pursed lips? Is that supposed to be sexy now? It's a major turn-off for me, that's just my opinion.

I took my profile picture while sitting in my car last week. I was waiting a little bit before going in to an interview that I was a bit early for. The purpose of the picture taking was to send it to a coworker to make sure that I looked presentable. It came out well, so I kept it.

Because I'm often waiting on someone or thing - in the car.
So I'll do a pic.

I don't like selfies at all.

Maybe that's where they're living

For me it’s just convenient. I can utilize the rear view mirror when using my back facing camera and I’m more likely to look decent as I probably have fresh makeup on and my hair still looks decent.

The backgrounds aren't supposed to be the focal point.

I figure the car is not only good lighting, but also that is probably just after doing all their hair and make-up in their house. This is the final product before they go out and it might get messed up...lol

Because it's where I would rather be!

I'm curious, why don't you explain what is so interesting about your photos? Why is your background better in your photos than the interior of a car?

I have wondered that too--and the bathroom mirror---

Mine was outside ..taken by a daughter, while showing off the shirt she’d brought me from ..home, Oregon Took me forever to get a photo up, so I’ll not complain.

I don't take pictures of myself.

I'm pretty sure this is a function of having a rear view mirror and going "Oh I look ok I think I'll snap a pic".

Well.... Sometimes you just have to take a pic while going through a drive through damn it! Lol

Hahaha here's my car.

I rarely take car selfies. I don't drive. Lol.

I've never done this. I think I'll take one.

You've heard about why men want big guns or big cars, right? It's the same thing. Sort of saying "Sure, my John Thomas isn't big, but look at this car. That totally makes up for it!"

Because the current trend is for people to take pictures if themselves everywhere. This is a 'selfie society' so why should the car be exempt?

I guess its when they are alone or feel most comfortable or there posing lol.

Because we live in our cars, Mine is with my bicycle.