Why are there not generic phone OS installers? I am used to install and remove various operating systems in my PCs, even having several at a time.

Usually, to install a new OS means:


Burn the ISO to a CD/DVD/USB.
Insert it.
Boot. (Maybe you have to tweak a bit the BIOS first).


The only difference between PCs is its CPU architecture: x86, x86_64, arm, etc. Depending on that, you have to download one ISO or another. But I never have to worry about which graphic card, mouse, keyboard, screen, network card, etc. it has. The install wizard automatically detects that and installs the corresponding drivers. Sometimes, if they are not bundled, the installer also downloads them. Anyway, the key point here is that the ISO is always the same.

Now, many mobile OS come out: Ubuntu, Tizen, Firefox OS, the omnipresent Android, and why not any Linux ARM distro out there!. Sadly I have a random chinese mobile, that surely will never get official support in any platform, and I will never be able to test them.

But... Are not today's phones just tiny ARM PCs? Why is installation that different? Why do I always need to have a precompiled monolithic ROM specific for my phone model? Why not just a single "ISO" per OS for any phone that detects and installs automatically the needed drivers, just like always has been in the PC market?

Note: I understand the problem with privative drivers, but I remember Debian asking me for those when there were no libre alternatives at installation, and I could install them then or later. I suppose that workaround could be used in those cases too. Correct me if I'm wrong.
In short, it's because of the different ways that Android and MS Windows are distributed. MS Windows is a retail product, sold directly to PC owners. Therefore, Microsoft is responsible for making it run on PCs. PCs are standardized, and (nowadays) have a hardware-discovery mechanism, so Microsoft can design the Windows installer so that it can run on any PC-compatible machine. It's in their interest to make drivers for old Windows versions compatible with newer versions, so that they can keep selling new Windows versions to users.

There's also a technical dimension to this. Phones aren't just "tiny ARM PCs" as you suggest. The distinguishing thing about a PC is that it satisfies the PC specification, originated by IBM but now specified by an industry consortium. There are several more standards for how components of a PC interact, and they're what allows automatic driver configuration such as you find on a desktop PC. Graphics cards offer a VESA interface, which allows graphics to be displayed before a GPU driver has been configured.

All PC hardware nowadays offers Plug N Play, which allows the OS to discover hardware and configure the memory map at boot. Before Plug N Play, you had to change physical jumpers on each PCI card, and then tell the software what memory range, IRQ, etc. you'd set on the jumpers. And in the days before DirectX and its ilk, you had to do that separately for every game that wanted to use that hardware. Plug N Play came about mostly so that sound card (and other discrete component) vendors could make easier-to-install hardware.

In contrast, Android isn't a product that's sold to users: it's "sold" to device manufacturers. Phones (also tablets, STBs, media players, etc.) don't have anything like the PC specification, so it's not possible for Google to make an "Android installer" that can run on any device. Even displaying the battery animation when the phone is off requires it to load the Linux kernel with a framebuffer driver and a battery driver. It's the device manufacturer's responsibility to modify the Android source code so that it can run on their device, and the resulting Android firmware image belongs to the manufacturer: they don't (have to) give those changes to Google or anyone else.

The phone world doesn't have these kinds of standards, because there's no demand for it. There's no one trying to sell phone components that you put together yourself. No one in the phone industry is interested in making an OS that you can install on any phone - only the likes of Ubuntu and Mozilla, who are outsiders with no contribution to hardware standards.