Why the backbone of polylactic acid's repeating unit contains only one oxygen?

The monomer unit of polylactic acid is $\ce{CH\_3-CH(OH)-CO\_2H}$:


[![https://en.wikipedia.org/wiki/Lactic_acid](https://i.stack.imgur.com/DjSLn.png)](https://i.stack.imgur.com/DjSLn.png)


So why the backbone of the repeating unit is $\ce{-C-C-O -}$:


![](https://i.stack.imgur.com/xAhGr.png)


and not $\ce{-O-C-C-O -}$?


