The priority of a big company is to get a "good enough" person as quickly as possible. They have tons of applications, and interviewer time is expensive. Naturally, the process is geared towards minimizing false negatives, and getting a reasonably good hire in the minimum time. The current process works well for that. People who clear these interviews are usually not terrible, and are good enough. There is no incentive for a large company to do anything differently, unless they are looking for a very specific skillset, which is rare.