CommentRe:Now when voltage regulators fail (Score 4, Informative)188
The other rails are a throwback to the 1980s and 1990s when TTL logic ran at 5v and later 3.3v. Today's logic don't run on 3.3v* or 5v or 12v; components run on less than 2v (sometimes less than 1v) and are highly sensitive to crosstalk and power droops.
It is simply not practical to build a PSU that supplies hundreds of amps on tiny, sometimes variable, voltages, on isolated rails with shielded cables for every component. As a result, devices today treat the PSU as a sort of mains power and implement their own converters and regulators to provide each component what it needs.
Motherboards have VRMs, GPUs have VRMs, drives have VRMs... a modern PC is full of them. So why bother with an assortment of legacy PSU voltages if they're just going to get reconverted anyway?
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(*This is a bit of a simplification; there are plenty of native 3.3v components in a modern PC but they tend to have trivial power requirements)