Google has been using rack-level UPS for many years, so I'm curious to hear why you think that is impractical specifically in large datacenters.
https://cloud.google.com/blog/products/compute/google-joins-....
https://cloud.google.com/blog/products/compute/google-joins-....
In third party data centers you're co-located with other customers that have mostly comodity server hardware which won't have those options, they generally (at least in my experience) all provide UPS power as part of the facilitiy so you have it already and something like a fire if it did happen, would impact lots of other customers who likely don't have redunancy between data centers or fire compartments.
Additionally depending on your exact setup (it's possible you may have dark fibre directly out of the facilitiy, but often not) you're likely to be uplinked through powered equipment at the same facility, that wouldn't have the extra UPS power either.
Plus as others mentioned, the loss of cooling is the biggest problem. In fairness maybe that would buy you a few minutes for the kindof automatic switchover they talked about. But that's quite a bit of co-ordination to have your own power, to not rely on anyone else being powered and then to be able to co-ordinate your shutdown before the room gets too hot - and not be at risk of overheating the room for others.
Plus there are safety issues like emergency power out for fire fighters etc which again, I'm sure Google could deal with at a facility-wide level, but if you're a small fish in a much bigger data center its harder to co-ordinate that kindof thing.
So while I'm sure it's all possible, there are obvious headwinds in many directions.