Why the Retina Display is just showing 1680x1050 pixel?

I love my M1 Air, but im a bit confused about the Retina Display resolution.
Its announced as 2560x1600, but in the Display settings, i just can choose 1680x1050 as the highest rate.
i thought about, that the pixel are almost double in size, to have a nice smooth picture.
I have a two port TB hub connected with an HDMI port.
When a 4k screen is connected, i can choose higher resolutions, but not without it...