Indeed, I suspect that (as with low end routers) there are substantially fewer distinct designs than there are brand names and rebadges, which would make 3rd party firmware easier. On the minus side, in areas where rebadging is the rule it can be a real pain to ensure that you get the same hardware reliably: if your vendor is slapping their badge on one ODM's cheapo board today, they could(and not infrequently do) switch to slapping the same brand and model name on an entirely different board with approximately similar capabilities tomorrow.
This is hardly unique to IP cameras and DVRs, the OpenWRT hardware support wiki is loaded with examples of routers that sell under the same model name and number but are totally different internally(as well as ones that are sold by completely different companies, and internally identical) and USB peripherals, the nastier PCI/PCIe cards; and even computers that aren't associated with 'business' brands that promise image stability will sometimes swap chips without notice.
I'm not sure if it's a specific business decision, or some sort of culture/language thing; but these sorts of situations always struck me as an opportunity for some entrepreneurial type in China to simultaneously distinguish their product(albeit for a limited market) and get some software development and localization done more or less for free: Western FOSS tinkerers love cheap hardware to play with; and while some established vendors play fairly nice, the combination of 'IP' enthusiasm and a desire to tie hardware to various cloud services and app stores often limits how cooperative establshed western brands are with what the FOSS people want(eg. Intel recognizes the value of having non-awful, in-kernel, drivers for their NICs and chipsets and stuff, since Linux is serious business in the server market; but takes a "your motherboard comes with cryptographically signed UEFI, and you'll like it." attitude). If you have the necessary contacts and business relationships with hardware manufacturers, access to datasheets, etc. you could position yourself above the other rebadge outfits by assuring that your product has a known, stable, chipset and hardware design inside; and by being as helpful as possible to OpenWRT or an analogous effort; and both reap extra hardware sales from tinkerers who want to be sure that they are getting hardware with good 3rd party firmware support; and have the option of basing your official firmware on the 3rd party work; rather than the in-house atrocities that so often ruin otherwise decent hardware.
I don't doubt that it is harder than it looks; and my Mandarin isn't remotely good enough to try; but if I had hardware that offers excellent value, ruined by firmware that is utter crap, it seems like this could be a win-win.