I always see them listed together actually, sounds strange

Which cars? All the ones ive seen have both, even if they only advertise one

I haven't seen 1 car that has CarPlay and no AA but I'm also not looking.

Where do you live? In the US it's much closer to 50/50 market share. And Apple is much higher among the affluent who are buying new cars.

i have a subaru and it came with android auto support (and air play support)

https://www.engadget.com/2197854/why-car-manufacturers-ditching-android-auto-explained/

So they can spy on you more. They don't want to be locked out of certain data streams and people who use android give up a ton of information to Google and other apps. But crucially a lot of that data isn't shared with the head unit and the TCM.

That means those automakers don't get that telemetry data and they want it.

Apple users are more likely to pass on a vehicle that doesn't have Apple Car Play than Android users are to pass on one that doesn't have Android Auto.

Not a car guy. I'm actually a phone guy.

Basically they are both the same product. Yes, one is Apple and the other is Google, but they are both doing the same thing. They connect to an external screen, which uses both WiFi and Bluetooth (and possibly a wired connection, though mine is wireless) to use the car's infotainment centre as a second screen. It's like taking a laptop and hooking it up to a TV. The TV doesn't really care if the laptop is a MacBook or a PC laptop running Windows or Linux. It's just a screen.

Granted, there is some basic handshaking being done behind the scenes, so it is possible to only support one of them. But most third-party models support both. It's probably easier to do that.

Also, it's super easy to think of Google and Apple as opposites, or enemies, but the truth is, they work together far more than their fanboys like to admit. Google is the default search on Apple products, and Apple Music is available for all Android devices. Apple TV is available for all Android tablets and TVs, and I think there's a way to get it working on Android phones, but I'm not sure. I have a Google TV (by TCL) and it actually has AirPlay. When you go into the settings, it's like iOS as far as how it looks, but the rest of the TV's settings looks like classic Android (circa Lollipop/Marshmallow, so like 10 years ago). And you don't even need an Apple device to get an Apple account, though I think you do need one to get an @icloud.com email address, and then migrate your account to that. You can sign up for an Apple account for free and you can use any email provider you like, including Gmail, and you'll need to do that in order to use their apps. Also, a lot of Android (phone) guys use an iPad if they want a tablet, for two reasons. One, Android tablets suck, and the ones that don't aren't really well known, whereas you get an iPad, you know it's a good one. They don't make ones that aren't good. They make a good one, a smaller one, a thinner one, and a pro one. Some come in different sizes. Two, best of both worlds, you can use Apple apps in the App Store on the tablet and Google Play stuff on the phone. My wife has an Android phone and an iPad and we have two Macs, an Apple TV, and a HomePod Mini. It's never been an issue for her.