Why don't PCs have built in battery backup?

I poked around the web looking for a PC (not a notebook) case or power supply that comes with its own "built in UPS" or "built in batter backup" of sorts. I own various UPS (Uninterruptible Power Supply) devices, and I find myself looking at them thinking - why?

Yes, I know the obvious reasons why you should use an UPS (in case power goes out, or a surge or brownout occurs), but the "why" that I asked myself was why don't the PCs include a *small* battery unit internally to at least handle a few minutes of uptime and protect against surges/brownouts without the need for an external device sitting there taking up floor space. It seems to me it would be much more efficient if a rechargeable batter was placed inline with the PC's power-supply (on the outgoing, DC side of things) where if AC power was dropped, it would provide the necessary 12V and 5v (or whatever other low voltage requirements) power to the computer. In addition, a built-in unit like that could easily interact with the motherboard and "inform" the computer (and/or OS) that A/C power was compromised.

With today's Lithium-Ion batteries (like notebooks use) the battery unit could be quite small and easily power the computer for a while. Or, even less expensive lead-acid batteries (like UPS's mainly use) could be incorporated.

I'd also go as far as to say that PC manufacturers could then get together with external component makers (like Flat-Panel LCD makers), to work on a "standard" for low-voltage external device connection that could also make short-term use of the PC's proposed internal battery. Have you ever noticed all those AC-to-DC adapters for your LCDs, some external hard-drives and CD/DVD-ROMS, etc? Seems ridiculous to me. I know that USB allows for some very low-current connections, but there is probably a good case to be made for higher-powered external devices to have a way to plug into my (theoretical) PC with built-in-backup-power.

The main thing would be to just have enough time for the system to perform an orderly shutdown if needed. I think there have been manufacturers of the PC power supply that I envision, though I could not find such a thing on the web.

Just thinking as I write my technology blog entries again, and hoping for improvements in the PC/computer world.