Mobile users are generally unhappy with limited amount of RAM in mid-range devices.
System settings always report the RAM that is available to user processes, but not all of the physical RAM. It do not report memory used by Android's system-level processes and by GPU which uses shared memory.
They often search for a solution to increase the amount of RAM available in mobile devices.
However, it is true that you can't upgrade it. Most smart phones are designed as a System-on-Chip(SOP) method, which means that the CPU, RAM, GPU, device controllers and other hardware in a single chip.
Updating RAM in such system means replacing a whole lot of hardware in it.
If you managed to change hardware, then it is highly unlikely that the existing drivers will work with that hardware. Also, you need to recompile Android with your new drivers.
Android is an operating system like any Linux distribution, or Windows. Android OS can run on x86 hardware, or on ARM boards too.
Additionally, supporting upgradable RAM will greatly increases the amount of space for the internal elements and will ultimately increase the size of the phone and it weight.
Existing applications in device may also be incompatible with newly upgraded RAM.
So, it seems impractical to upgrade the RAM of an Android device due to hardware and software restrictions.