In the "old days", consoles were pretty simple affairs compared to what we have today - largely manufacturers relied on the cost of the physical cartridges to ensure lock-in and control over the available games on their systems, along with copyright control over the documentation required to properly code against the system.
Of course, it was possible to spend some time with a hardware debugger and discover exactly what the system was doing, and you could design your own compatible cartridge to use the console - so long as you did not include any copyrighted material in your cartridge hardware or software you were fine and you didn't have to pay any licensing costs to the manufacturer.
Times have changed however, both in terms of how complex consoles are and in terms of the legal situation surrounding them.
Modern consoles go a few steps further - they include cryptographically locked boot loaders and firmware, which allow the console manufacturer to require content to be digitally signed with specific keys, keys only the manufacturer has, before the console will load the content.
There are ways around this, you can remove the protection in the hardware by replacing it, or you can circumvent the firmware with new firmware which doesn't require a signature check, or you can in some cases include a signed file in your game which is checked.
The problem with these approaches is that invariably they require you to distribute code or a derivative of code which is copyrighted by the console manufacturer (because the firmware is more complex than just the signed bootloader and you really don't feel like writing something as complex from scratch, or because the files you need to include are copyrighted and signed by the console manufacturer and thus have to be perfect copies to match) - and this is where the main legal claim comes in.
Not only does the manufacturer have a copyright claim against you, but they can also bring the Digital Millennium Copyright Acts clause regarding circumvention of access control measures - which you would be doing in this case.
That is why its difficult to produce unlicensed games on modern consoles. Theoretically it can be done - you can replace the firmware with your own completely written from scratch or find another way to distribute your content which doesn't involve infringing on the console manufacturers copyright, but the manufacturers make this extremely difficult.