Hollywood disagrees.
https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2F...
I'd agree with you if Tesla had done anything meaningful prior to Elon's involvement. Using your analogy, Tesla wasn't even a little backwater restaurant that got investment—it was just a couple of guys who thought it would be neat to make hamburgers.
If you exclude the six months spent looking for capital, Musk was there from the very beginning. Before Musk, Telsa was just a couple of guys looking for capital. Musk WAS the capital. He was Tesla's very first investor and has always been Tesla's dominant shareholder.
Did the incorporated entity do anything consequential in those six months—other than searching for Elon Musk?
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)
Never put off till run-time what you can do at compile-time. -- D. Gries