Why is it "so called"?
What do you believe it ought to be called?
It's really the "Linux OS" (or possibly "GNU/Linux OS") with a "Chrome UI".
Microsoft Windows is both the OS and primary/default UI, so many people think the UI is OS, and carry that thinking to the Linux world. Google is perpetuating this by calling their UI replacement the "Chrome OS".
As an aside, I can’t remember the last time I had to look at the 'guts' of a linux system to make anything work. The biggest problem I've had is that the compatible programs often don't use the same name a the MS Windows equivalent, so it's hard to find in the repository without a google search. Some are trivial: Tor is tor, Steam is steam. Some are simple: LibreOffice replaces MS Office. Some are absolutely esoteric: MS Windows Networking is a Samba (MS calls it 'Server Message Block' networking, which they shorten to SMB, which is is pronounced 'samb' or 'sumb', which linuxers make 'samba')? Fortunately, the distro I use (openSUSE) has a good search ability in it's software installer, and many developers include the 'just like * on Windows' in the application distribution.
As to reasons why to use Linux, my favorite is the 'start menu'. I know, MS wants everybody to just forget it ever existed. That's because, on Windows, it's an unholy mess, every program simply dropping itself at the bottom of an ever growing list of unorganized "all Programs", with each installer creating it's own sub-menus with their own naming scheme, and the only automatic organization is by the name of whatever is created at the top level by the installer, which might be the program name, might be the company name, and might be the program type. And if you attempt to reorganize it yourself after the install, you end up with leftover bits when you uninstall that lead nowhere.
In the KDE UI, it's
automatically organized by program type (Education, Development, Games, Graphics, Internet, Office, etc.), then by application function (Web Browser, Chat, News Reader, netPhone, etc.) If only one such app is installed, it's right there; if you have more than one, they have a second sumbenu. So, the menu itself always points you to where you need to go to find the program you want. You do always have to go into one submenu, and rarely two, but the only place I've only seen a third submenu is for system configuration (control panel). Some programs show up in multiple locations (steam (the game store) is listed under "Games->Steam" and "Internet->Data Exchange->Steam"), but only if they actually have multiple functions, and that generally makes them easier to find.
Another, minor improvement is the act of shutting down. In my last job, one of my daily tasks was to ensure that all the computers were shut down. It would be nearly impossible to count the number of times that I walked into someone's office only to see the "Do you really want to shut down?" prompt sitting at the center of the screen. Yes, I like to have a confirmation dialog when I'm about to do something potentially massive like shutting down, or exiting an app without saving changes. But, on linux, that dialog has a countdown (user configurable) such that, if you don't cancel it, it will go ahead and shut down like you already told it to 30 seconds later (user configurable). So many MS Windows systems sat there, asking "really, boss?", fully powered on, simply because the user didn't say "yes, you stupid machine, I really wanted you to do exactly what I just told you to do!". As I left up to 5 hours after some people, that was a lot of wasted on-time. A minor, almost trivial thing, that Microsoft never bothered to implement.
) Linux will the go to OS for people who do not want to spend hours on their computer and for those who do.
