No reason, apart from they're competing with cellphones these days. My phone can record until its storage is full, in 4k.
I'm not a professional, but I am buying my first proper camera (I went with the R6) right now. A cine camera definitely wasn't on the cards — I want a camera to learn still photography, make youtube videos, and sometimes record shows for amateur theatre. While I do wish the R6 had a 4k mode that didn't overheat (like the R5 does), the 30 minute limit is actually more annoying to me, because my much cheaper multi-purpose device (cellphone) can do it.
I'll probably buy an external recorder. But I feel annoyed that I have to
Welcome to the world of photography in that case! Hope you stay engaged in here and we can help you s time goes on!!
Few thoughts based your statement:
Congrats on choosing the R6. It will be an absolutely excellent camera for you and ME! I have one on pre-order. If your goal is to learn photography, the R6 is the best choice you could have made to do that. You will be very, very happy. The EOS Cinema line would be overkill and would not meet your goals.
If recording YouTube videos and basic live events is what you want, the R6 is flawlessly perfect. The overheating stories are a non-issue for you.
You will be recording in 1080p24 or 1080p30 for Youtube. Even in 4K24 or 4K30, you'll be just fine. The camera will NOT overheat in these normal, everyday recording modes of standard speeds/framerates.
Ignore the "overheating" stuff. None of it applies in real world routine situations. Only in very specific, comparably very rare instances.
If streaming/recording to YouTube, you'll probably want to invest in a good HDMI video capture device anyway. They run about $100. This will let you direct output clean HDMI to the device to record or live stream straight into your computer. This is the case with ANY camera.
Should you wish to record internally in camera, you may! But if you will do YouTube regularly, I would advise you get the capture card. The $100 is well worth it to make your life a whole lot easier.
For theater, you obviously need more than 30 mins uninterrupted. Yes, buy an external recorder or buy a dedicated Canon video camera. You don't need Cinema lines for this.
Phones dont have the same licensing requirements as dedicated cameras. Their sensors are also microscopic compared to full frame 35mm size like the R6. There is zero comparison in image quality and versatility. You're stepping up to a real camera. It's not an all-in-one entry level machine.
On the flip side, the R6 will have aa super sophisticated IBIS system. A $100,000 ARRI Hollywood camera does not have that. You have to strap it to a $10,000 body suit gimbal. So the phone to R6 comparison doesn't work when put into the right perspective.
Personally I've never shot a 30 minute video on my phone. They look awful when you play them back on a screen bigger than a few inches anyway. I can't say how long they go before overheating.
Anyhow, thanks for chiming in! Hope we can help out!!!! Best wishes to you in your new endeavor !! Canon Rumors forum should be a place where folks like you starting out can come and get help from us more seasoned guys and girls. Hope this helped!!