I don't have any inside info but I used to work in a phone support job.

My guess is that most people have an adequate support experience based on ASUS sales volume. If they were doing as badly on every support case as some of the stories posted here they wouldn't have the repeat business keeping their business alive.

So why are some support cases so bad? Based on what I saw as a support person in another company, the people on the front line answering phones are minimally trained and minimally paid. Why? Because premium support costs more and companies that offer it no longer exist or are relegated to boutique status. I'll use myself as an example: Last time I bought a gaming laptop I didn't even look at Origin PC or Falcon Northwest because they are expensive. I bought a Dell because it was much cheaper in the 1050ti range. So I'm part of the reason why premium support is rare because I choose not to pay for it. Most people are the same so we're all to blame.

TL;DR: Large companies target a "good enough" level of support because most consumers refuse to pay the extra margin that better support would cost.