[pd-ot] electric circuits in software

[hard off]

how hard is to model electric circuits by software?

what is so inherant in analogue and even digital hardware that makes
the sound hard to recreate by software?

recently i've been trying my hand at circuit bending, and the guts of
these machines are just a bunch of resistors, capacitors and
transistors (etc)

surely it isn't too hard to model these by software?

obviously "rebirth" was the most famous analogue modelling software. 
but why isn't there software that just lets you join a bunch of
resistors, capacitors and transistors to make pretty much real
analogue synths???

why isn't every analogue synth ever made already coverted to a coded equivalent?