It's not quite as easy as it sounds - but Netflix are at least doing something, and starting with their original productions is a logical place to go, because they have complete control of the master copies and can commission an Audio Description soundtrack to run in sync with that.
Adding AD to third-party content is trickier, because for the most part it's not Netflix's content to audio describe in the first place. If the distributor has AD assets available, it's then a complex process of aligning those assets to the pre-existing copy of the asset which Netflix is using. That's not always just "drop and go" because the versions won't necessarily align, or even contain the same visual material (e.g. Cut versions, syndication edits, director's cuts, special editions, etc.)
At the same time, "just use the description from TV" is not that straightforward either - here in the UK, the audio description is almost always produced by the television channel showing the programme, and NOT the programme maker/distributor themselves. Accessing that content now becomes more complicated, both proceedurally and contracturally.
It's not trivial - but at the same time it's not impossible. And for all the stick they get, Netflix actually leads the way in accessible content - there are vast numbers of video on demand services which simply do not even support access technologies in the first place. (It's a bit better in the US, where the FCC lays down rules about availability of captions on online video, admittedly.)
But especially here in the UK, almost no VOD services support even basic subtitles, let alone multiple audio tracks. In contrast, Netflix is a breath of fresh air - they actively seek out and obtain subtitles/captions, and now AD, from the third party producers that they licence content from.
They deserve praise for their attitude in this area, in my view - they put some of the largest media companies in the world to shame. That might sound excessive, but I've lost track of the number of major, major media companies whose reps pass on the company line that accessible VOD is "not possible at the moment", when the reality is that it's perfectly possible, they just choose not to do it or to make it any kind of priority.
With that kind of competition, Netflix is an easy number one in the field. They know their stuff and they're doing what it takes - I've got nothing but respect for them.