You could do extensive closed-course testing with realistic conditions and then pay to have human drivers watching & ready to take over at any time. The problem is that this costs money and takes time, and Tesla was hoping not to do either.

Closed course testing to compare with humans would require about 800,000,000 miles of testing (10x length of average travel between fatalities). And that would just be for one version, every update would require that, and updates are about 100x internal version changes per year depending on how you count additional ML annealing.

That's about 80B test miles per year at about a capital cost of $0.50 per mile.

For a company that, say, spends ~10% on R&D, it would require the company be about a trillion dollars, which only Apple, etc. have reached.

Every other system has rigorous testing with trained safety drivers, planning interventions and working in short enough shifts to remain attentive.

Tesla does some of this, but then releases fast to the public... So far it has had some concrete consequences when they pushed this testing responsibility onto unaware people.