Reverse the perspective for a moment.
You are a hardware manufacturer and game publisher. You have a premium product, what you consider to be the best in the world. You have developers lined up to use your product. Not just any developers, you have the best and the brightest, the biggest companies, all willing to pay you in order to develop products on your system, and then pay you more for every product sold.
It makes some sense to open it up to people who have some experience developing products elsewhere. After all, if you can get these people who are already making games on their own, you might be fortunate to get people before they make exclusives for other platforms. Of course, the group collectively also produces lower-quality products. Since you don't want your high quality premium product to be associated with low-quality goods, you can use your certification process to ensure they meet the quality standards.
But why on earth would you open your premium product to anybody? In particular, the "anybody" who are absolute beginners, who collectively don't even know what they are doing, who have no skills and are statistically worse than the lottery when it comes to making money. That group has a demonstrated history of publishing bad goods, a demonstrated history of giving companies bad names.
Look at Microsoft, and how they were burned on that. Microsoft had been bitten badly on the topic with DOS and again with Windows, they learned their lesson before entering the game console arena. Anybody could create device drivers and add-ons, and 'anybody' frequently did. When systems would crash people would blame Microsoft and blame Windows, but in fact Windows wasn't the problem, it was the shoddy third-party drivers and programs. It took Microsoft over three decades to shake free from that, eventually introducing enormous barriers between customers and the ability to install drivers from third parties, big scary warnings to users before they installed them, plus modifying how crashes were handled to ensure that Microsoft and Windows were clearly not the product at fault, but even still today when shoddy third-party programs crash on Windows people still blame Microsoft and Windows for the crash after they've done everything possible to stop it. They don't want that to carry on to video games.
Sony's video games division started out, and remains today, a small portion of their business. They know full well from their big product lines how effective shoddy merchandise is used to break into markets, and how damaging shoddy merchandise can be to their brand even when their product is amazing and the shoddy competitor is the the problem. and they've been fighting it since the 1950s. Inferior brands of transistor radios in the 1960s meant people would blame the highly popular Sony models even though Sony had nothing to do with them. Inferior Walkman knock-offs in the 1970s and 80s meant people would complain about Walkman products even though the defective product was a cheap competitor. Repeat again with inferior CD players after Sony worked so hard to develop the format with their partner Philips. Inferior knock-offs and components in semiconductors, in televisions, in so many products. Piracy in their Sony Music and Sony Pictures divisions. Sony has spent nearly 70 years living what happens when they release the world's best premium product in a marketplace only to be devalued due to completely unrelated shoddy third-party products and knock-offs. Twice they nearly collapsed to bankruptcy due to the problem. Even though the company has diversified a hundred times over and games only make up about 5% of their business, they will fight with every tool they've got to ensure their premium products and premium brand is protected against any external entity.
Nintendo learned their lesson the first time from Atari and Coleco's Telestar in the 1970s. Their Color TV-Game wasn't the most popular of the era but still sold millions of them and saw what happened when their competitors had unauthorized cartridges. Nintendo learned the lesson again with the NES and used lawsuits effectively to prevent people from tarnishing their brand. They never forgot the lesson, and won't let their name be tarnished that way again.
The door is open wide to anybody who can (1) create a high quality game, and (2) bring the resources to bring a high-quality game to market. If someone comes to them wanting to be a potential business partner and isn't able to create (or can't be bothered to make) their own high quality product to contribute, why should the company partner up with them?
If the game developer can create a high quality game then they're welcome to partner up. Because in that case Sony, Microsoft, and Nintendo do see the potential for a good business partnership. Even if the game is not world-class quality, it can add value. But if the game developer has nothing they can contribute, the console company has nothing to gain and everything to lose by partnering with that developer.
