It surprised me as well. But apparently, it's up to the authors (or their institution) to choose.
> From January 2021, authors submitting primary research articles* to Nature will be able to choose to publish their work using either the traditional publishing route OR Open Access.
> *Non-primary research (e.g. Reviews, Comments, News & Views) is not eligible for Open Access and is only published using the traditional publishing route.
From the list of latest articles[1], you can see it's indeed not the most popular option. Only one in seven articles from the last few pages is OA.
The other OA journals still cost money that needs to be budgeted, i. e. not something you would pay out of your pocket. For example PLOS One charges you $ 1749. I guess the prices for publishing the articles may actually converge on a fair price for the reputation associated with the journal.
> From January 2021, authors submitting primary research articles* to Nature will be able to choose to publish their work using either the traditional publishing route OR Open Access.
> *Non-primary research (e.g. Reviews, Comments, News & Views) is not eligible for Open Access and is only published using the traditional publishing route.
https://www.nature.com/nature/our-publishing-models
> The APC to publish Open Access in Nature is €9,500
https://www.nature.com/nature/for-authors/publishing-options