You can't mount them because there has to be some software to communicate between the phone and your computer. With the Phone the full software stack needs to be involved as it needs to take into account what happens when you mount the device when it's running. Also there are often multiple partitions on the phones, a raw mount would show up all the raw system files in the computers File Manager which would be bad and things like Windows would probably try and format the Linux partitions as it can't understand them. On newer devices you can't even do a normal USB Storage mount but you have to use MTP as they use internal storage which still needs to be accessible while mounted. Technically it would be possible for them to make some kind of failsafe raw mount mode when it's plugged in while powered off (or started with some button held down), but they haven't, they do provide other methods of recovery instead. Also the firmware generally needs to be installed somehow, there are scripts and so on that run to set up things. If you have totally bricked the phone then there isn't much you can do.
But having said that flashing modern Android phones isn't too risky and actually bricking it is hard.
Firstly there are actually several images that can be flashed:
- Boot loader (HBOOT)
- Radio firmware
- Recovery Firmware
- Rom Firmware
Older devices did have some issue when flashing Radio/Boot. Specifically if you flashed them in the incorrect order and had mismatched versions you could actually brick the phone. But I believe that is mostly a problem with first/second generation Android phones, I haven't seen it being an issue on newer phones (also you probably shouldn't flash a radio of a different region as you might loose special features, like the 'HD Voice' for Australians).
Now days, in order to actually brick your device you would have to be totally unable to enter
HBOOT, Recovery and the main Rom. If you can enter any of those it should be possible to flash firmware from them (although in some cases they might have security enabled meaning you are limited to flash the official stock images, but you can generally break out of them and flash whatever you want). HBOOT allows for fastboot flashing. Recovery and the main Rom both have the flashing binary in them.
Finally if you did totally manage to brick it, you can look at using JTAG to reverse it. That probably does require soldering a cable onto the phones mobo though as well as a (cheap) cable. And some knowhow.