The trick is, not everything else scaled proportionately. The gate lengths (mostly) stopped shrinking at around 34nm but other things kept shrinking, so the overall transistor density kept going up.

(And that assumes planar transistors. Things like FinFET or nanowire which make the transistor structure 3d instead of 2d further disconnect the gate length from the achievable density.)

> Historically, the process node name referred to a number of different features of a transistor including the gate length as well as M1 half-pitch. Most recently, due to various marketing and discrepancies among foundries, the number itself has lost the exact meaning it once held. Recent technology nodes such as 22 nm, 16 nm, 14 nm, and 10 nm refer purely to a specific generation of chips made in a particular technology. It does not correspond to any gate length or half pitch. Nevertheless, the name convention has stuck and it's what the leading foundries call their nodes.