I believe it is a 3 prong issue.
#1 is the frame encoding method, is it intra-frame (All-I) or inter-frame (IPB). Intra-frame is where each frame of the video is independent of the others, obviously high quality, implicitly requires higher bit rate, Inter-frame is where an Intra-frame is taken, then the next 2-5 frames are built based on changes to that initial Intra-frame, this is lower bit-rate, allows for higher compression. Most (if not all) dash cams, and even action cams, are inter-frame.
Progressive is the answer, where each frame is a complete picture in it's own right. That's the only way you'll ever get a perfect still as it's not a mixture of 2 frames unlike interlaced. Full frames though contain twice as many lines and thus data as interlaced though.
#2 is the bit rate, less bits is less data. Frame rate will also play into this, a camera shooting 1080p60 at 20Mbps will have a worse image than one at 1080p30 20Mbps because there is more data per frame at 30fps, not hugely as the encoding schemes are typically variable bit rate. The 30fps will have more motion blur though as well, obviously.
True because with one you're trying to pack twice as many frames into the same amount of compression. However, many cameras up the bit rate for 60fps modes helping to alleviate this. Also, it's a matter of compression losses vs speed blur.
#3 is the field of view of the lens. In combination with the encoding method and the bit rate, the wider the FOV, the more image you are trying to squeeze into the frame, so data will be lost as encoding tries to keep the file size under control. Particularly small detailed items with similar colours that are close together, like the characters on a license plate.
True it doesn't help from that perspective. But the real issue with wide lenses is that non professional lenses tend to be non linear across the face of the lens, so you get increasing amounts of distortion and thus loss of sharpness at the edges. The answer here is a low distortion lens. However, considering the low distortion lenses cost around $100-120 retail on a M10 screw (based on the Ribcage sites lens selection), that's probably $50-55 trade and so it adds to the cost of the camera. I do believe it's possible though and that at least one manufacturer is looking into this following suggestions I made about putting an M10 socket onto the front of dashcams to accommodate this. It's important to understand though, that any resulting cam is unlikely to be low end and $50-100 to buy. Anyone who's ever put a quality lens on a cheap SLR camera, will know just what a difference a quality lens can make. Conversely, you can buy the highest end camera body, but stick a consumer lens on it, and you won't get magazine quality shots. The lens is one of the key components. However, goods glass doesn't come cheap!
I don't understand why a company doesn't make a high end camera that is 1080p60, 1440p60, or UHDp60, with a 50Mbps or higher bit rate, All-I frame encoding, and selling it for like $350-$400.
If you take a look in some of the sub forums on here, then you'll find there is experimental 3rd party firmware with rates in excess of 50mbs. Some of the action cameras also have data rates of 100mbs. However, pure data rate alone isn't the answer. The issues are a combination of data rate, lens, sensor, processor, metering and cost in my opinion (might not be a comprehensive list as quickly put together and I am an amateur not a camera expert).
Also, so far as cost goes, I doubt you'd sell many Dashcams at that level. Personally I see the top range being $200-250 or £200-250. That would be top end and so probably not huge selling but I would imagine still viable as it's not beyond the top black box style dashcams now. Beyond that in price, I think anything you'd get would be very niche. Personally, I'd trade some features for image quality.