Based on scale of sizes, transfer methods, bottling methods, homebrew picks up a whole lot more oxygen than commercial beer. At my brewery, our cut off limit for dissolved oxygen is 100 ppb. Most of our beers are in the 20-40 range. Towards they end of their shelf lives they start to show the signs of staling, cardboard, sherry, etc as mentioned above. My own homebrews I've tested in the 1000s. Maybe other people are better at it than me, but I know my stuff was going bad within like a month. Homebrewers (except for those with conicals and pumps and CO2 pressure, basically nano-pro gear) will never get down below 100ppb.
I recently stopped transferring my beer to secondary, which is one less time opening the carboy, one less time running it through a siphon, etc etc and it has made a huge difference in my beer quality, so even that reduction was worth it. I may still be at 500ppb, but I'm half what I used to be.
To answer your original question point blank, people are obsessed with it because it makes such a huge and noticeable difference.