And if obtaining root access trivially is important to an Android user, they will choose their device accordingly.
So how does one who has been given a hand-me-down device, such as my cousin, go about that? Sell the device on Craigslist and buy another?
Nexus devices don't require some exploit to be found to achieve root... it's a very straightforward process.
Root on a Nexus requires unlocking the bootloader, which in turn requires wiping the device. This means you lose all your data if you want to gain root at any time other than the day you buy a new device.
You can buy Linux devices setup as kiosks that lock the user out of root.
The difference is still that GNU/Linux PC owners are expected to have root. All major distros either ask for a root password or put the first created user into a "wheel" group (which has sudo rights) during installation or the first boot. So there is root by default on the "typical" GNU/Linux PC. Reminding the machine's owner to establish and keep root access is the rule, not the exception as it is on Android. Besides, there are plenty of things that don't require root on GNU/Linux but do on Android, such as installing fonts to a single user account.