Let's concentrate on your statement for a minute:
"why we say it linear and time invariant."
and specifically on the word: "it".
The definition of linearity does not say that "it" (the resistor itself)
is linear really, it says that the "voltage current relationship" is
linear for the resistor.
If you try to work it out using the voltage and current it may make more
sense.
Adding resistors in series would not be a linear process but rather a
discrete process.