The tech isn't ready yet. A chip that can handle two 1080p video streams at a good quality doesn't exist or is very expensive, or maybe one hasn't been made that's low power enough or durable enough to withstand automotive use.
To expand on the above comment by
@Gibson99, the way I understand the dash cam manufacturing world, 1-ch and 2-ch dash cam systems have the same common objectives, namely, recording front or front and rear while you travel, with a second (and optional) objective being recording front and rear while parked (i.e., having a "parking mode" feature).
However, for a manufacturer to make the jump from a 2-ch system to a 4-ch system is not as simple as designing by extrapolating the 2-ch dash cam system into a 4-ch system. Manufacturing a 4-ch dash cam system, even as a 2x 2-ch systems, implies that manufacturers must offer the parking mode feature. This is not an implied feature when a manufacturer offers a 1-ch or 2-ch dash cam for sale, but, IMO, it is when offering a 4-ch system.
This implication follows from the fact that (unless most consumers travel at abnormally slow speeds in the city [which they don't] or are interested in recording distant scenery in the open country [which they are not]) the "panning" effect on the 2 side-view recordings renders useless virtually all recording originating at those 2 side-view cameras. This results from the optical effect of
the speed of travel by a system that is recording at 30fps. (You can witness this undesirable panning effect with a home camcorder when it is panned too quickly from one part of a room to another.) The result of this is that the 2 side-view cams would be most useful when the vehicle is stationary, i.e., when it is parked; thus the need for the parking mode offering to make side-view dash cams useful.
That said, true, long-term parking mode (aka, "buffered PM") itself implies hard-wiring the DVR unit. But hard-wiring itself implies considering a host of other issues. Some of these issues are eventual vehicle battery voltage depletion, wide swings in Wh from seasonal temperatures, overheating of DVR battery, need for super capacitors and dedicated battery banks. I am sure there are others. Faced with all the challenges, would you rush to offering a 4-ch DVR if you were the manufacturer -- and especially knowing that the end product had to sell for less than some certain magic dollar amount to make it attractive to consumers to buy?... Its a big expense in market research as well as engineering R&D to end up not making a decent profit at the end.