Why does R not auto-complete data frame variables when subsetting

Question

Using the data frame mtcars on RStudio.

Say for example I want to subset mtcars[mtcars$cyl == 4,]

Tabbing after mtcars$ will provide a drop down list of variable names in the data frame.

Tabbing after mtcars[mtcars$ does not return the variable names.

Why does this happen?

Answer 1

it will if you add a space:

mtcars[ mtcars$

otherwise your expecting r to look in something called mtcars[mtcars not mtcars...