In a situation where you have that many disks and fully redundant storage, the lower purchase cost may win out over better reliability in terms of total cost to the business. It's a very different equation if you aren't working in the same parameters. No-one is saying that our purchase decisions should be the same as theirs - they are just being kind enough to show stats over thousands of drives, which most of us couldn't afford to gather, so we can use that in making our own decisions.
This is similar to Google running servers above recommended temperature and wearing the cost of higher failure rate because it's cheaper than running cooling to keep the servers cooler and more reliable. The cost in convenience of doing the same with your desktop or gaming rig probably isn't worthwhile when you're going to have it in a room that has to be comfortable for humans anyway. But it's still nice to see Google's stats on reliability vs temperature, and hear how it influences their decisions. Nerds are supposed to love this shit.