It might be because of brief spikes in power draw that a battery usually "smoothes out" but USB power can't handle. For the same reason, some laptops cannot run without a battery (most can).
The charge-discharge cycles needlessly put a lifespan on that component when I'd like to have it left on 24x7.
Same for always-powered laptops. With WFH, for 1+ year I've used my Thinkpad as a desktop, not bothering to unplug when fully charged, and now the battery doesn't hold charge for 10+ mins.
I don't know anything about how much "oomph" a dashcam needs as opposed to a smart phone or an uninterruptible power supply but I kno0w one of the bullet points in the marketing of the dashcam on my car was it uses a small capacitor as opposed to a battery and therefore it is safer to use it in a hot car.
I had a dash cam years ago that had a capacitor, and all it really was for was to make sure the camera could finish a write operation and close the video file gracefully to avoid corrupting the last segment. Cameras with batteries often can support recording even while the car is off without draining the car's battery.
IMO, the solution to the battery-in-a-hot-car problem is to have the battery be an in-line part of the power cable that can be placed in the glove box, outside the direct sunlight, rather than building it into the camera itself.
About 4 months after COVID hit and I started to WFH, the battery in my Pixel 3 started to swell because I had allowed it to basically live on the charger all day, constantly at 100% battery. Also, by that point, a 100% battery would only give me about 2 hours of screen time.
A few months ago, I got a Pixel 6 Pro, and I'm basically just charging it from a weak 500 mA USB port a couple hours each day, keeping the battery between 50-80%. I'd really like to set an 80% charge limit and just forget about it.
On my OG Motorola Droid, I once tethered my laptop to it and downloaded a torrent.
In 15 minutes, I drained 25% of the battery. IIRC, it had a ~2,000 mAh battery. That meant I was pulling 2 amps from it. This was back in 2010, when most phone chargers and USB ports were still only 500 mA.
If I had relied on USB power, it wouldn't have been able to power it.
Oh, and yes, the phone got incredibly hot during this time. I thought I was going to burn my hand when I picked it up.