Because of its treatment of capital. Other answers are possible.
This post draws heavily on the work of Pierangelo Garegnani.
I start with a (parochial) definition of economics:
"Economics is the science which studies human behavior as a relationship between ends and
scarce means which have alternative uses." — Lionel Robbins (1932)
The scarce means are the factors of production: land, labor, and capital. Land and labor are in
physical terms, in units of acres and person-years, respectively. They can be aggregated or disaggregated, as you wish.
But what is capital?
Some early marginalists, such as Knut Wicksell took it as a value quantity, in units of dollars or pounds sterling.
Maybe I should rather say, it is given in numeraire units.
Capital is taken as given in quantity, but