Why do we need a number of different terminal modes in Emacs?

[Andrey Lisin]

Hi everyone,

could anyone explain me why do we need several of terminal
implementations inside Emacs? Shell, eshell, term, ansi-term... And as
far as I noticed they all suck one way or another. For example, shell
cannot complete commands when you ssh to remote server inside it. Eshell
tragically cannot do a very simple thing - source a bash script. Really,
try to run `. some_script.sh' or `source some_script.sh' inside eshell.

So, am I right and we need them all just because one solves restrictions
of the others and others do the same thing for the one? Why then not
just to write some standard terminal implementation, that will just
works? Does it have something with cross-platform nature of Emacs? Or,
maybe I'm doing something wrong and all this stuff makes sense? Thank
you.