I do agree that unless you're particularly unlucky hardware-wise, an Ubuntu installation is more likely to succeed nowadays than fail. But they may be able to survive the installation, yet end up with minor to moderate problems that degrade the entire experience, which wouldn't be there, had their operating system been preinstalled, tested and supported with their hardware.
The question of hardware compatibility and other technicalities aside, the entire notion of the "operating system" being decoupled from the computer and installable separately is alien to most non-technical computer users, and the idea of installing a "third party" operating system is simply uninteresting; which is why I said "don't", not "can't". Why should they, when they already bought a computer with an "operating system", and it largely works?
My point is that the "download an .iso file from the internet, burn a CD / flash disk and install something called an operating system" model, while it's largely successful for the audience it's intended for today, is not way to advance the use of free operating systems in the broader computing ecosystem; striving towards more viable and widespread Linux-preinstalled devices is. But surely the success of the former will reinforce the latter, since a critical mass is needed.