Why only 45? And why water cooling?

It strikes me that building everything around room temperature or slightly chilled air is a strange choice. This is already 290K-300K or so, and now this is suggesting that things run fine at 320K or 330K?

I've wondered why we couldn't just design everything to operate around 200°C and just use free-cooling by pumping ambient air through. Why don't data centres look more like chicken barns? Do things melt? Are there more errors of some other type at high temperatures?