Yet less than 20% of the 80+ major vulnerabilities in confidential computing have been found by cloud companies, with the majority being found by universities and independent researchers.

It's not actually in their interest to look too closely. They know they are selling snake oil, and they do not care if it works. The only function of the technology, as far as they are concerned, is to be something they can market to customers who are scared at running on public cloud, and either they never intend to exploit it, in which case it doesn't matter if it doesn't work, or they do, in which case they don't want the vulnerabilities to be found and fixed. Either way, there is no upside to them for going spelunking.