One clue, is that there are many many possible structural isomers for a molecule of a given size. Abiotic chemistry makes a (sorta) random mess.
In contrast, biology uses molecular things. So you might see a few key molecules, but lots of them. Lower diversity.
As an example, go into a forest after a windstorm, wood of all sorts of lengths lying everywhere. But go into a lumberyard, and sizes and lengths are pretty standard: 8ft, and 12 ft.
Standardized stuff makes building things easier.