Still, you're saying that when a user buys a computer, it is their responsibility to go searching on the web for a free, reputable and safe anti-virus company to download software from?Yes it is.
Why?
Unfortunately due to antitrust concerns. I'm sure MS would love to bundle MS Security Essentials, but they fear doing so would likely end up in the courts.
Can't someone build a computer where you turn it on and it works safely the first time?
You can, although ironically its not in the best interest of most computer vendors. They get paid by McAfee or Norton to put a trial version on the boxes, which when expires usually results in a vulnerable box. Sure they could put a free version, but then they wouldn't get paid.
MS does directly sell computers, through their retail channel, with free antivirus pre-installed. AFAIK they're the only ones, but I wouldn't surprised if there is a small niche in the world of it.
Why should they care?
The reasons they should care are obvious. But they shouldn't have to care. I completely agree. It sucks. But I do think the visibility and sucktidiness of it is a lot lower than all of the other things that will suck when they unwittingly bring a ChromeBook home from the store.