Because just like broadcast television, they are sufficiently large enough to cover the costs associated with captioning the content. Netflix does have some content that is captioned on their streaming service, but frankly it's mostly a joke.
I am not sure what their process involves in acquiring the visual content that they stream. Netflix seem to have no difficulty in acquiring the audio tracks to stream along with the visual content. If Netflix is getting the content from the DVD's or whatever, it should be trivial enough to rip the subtitle / closed captioning tracks already present, sync it up and stream it as well. The physical DVD is captioned, yet the stream isn't. Netflix, along with a number of other content providers are basically saying, "Meh it's not worth our time to deal with it, so tough luck." Technically it should be trivial enough to do this but they are not. That's when the government usually has to step in. It's not so much a matter of preserving their profit, but doing the right thing and providing accessibility. Otherwise it's essentially tyranny of the majority.
The main reason why this lawsuit is necessary is because many online streaming services are essentially doing nothing or a very poor job of providing captioned content. Hulu for example, has a limited selection of captioned content within their catalog. This would not be so bad if they were more conscientious about monitoring their content. Sometimes they do not caption a given episode out of a captioned series. Apparently they have to "receive" it from the content provider. They do not check to ensure that the file has been received each time and that it plays properly. I've had missing captioning, captioning that was out of sync, content that indicated it was captioned, but no captioning, content that does not indicate captioning, but had captioning. If this had been happening with a broadcast television station, they would have been hit with a bunch of fines. That's why broadcast stations have someone monitoring it to ensure quality and delivery of captioned content.
Back to the point, streaming services are becoming more widespread, their catalog is expanding. They need to develop a scheme that simplifies handling and streaming of content such that captioning is automatically included and present from the content provider. That would ease the burden on Netflix, Hulu, and others. Netflix is the ideal target because they have one of the largest catalog of streaming content, and most of them were already captioned previously. In doing so, Netflix would have to sit down with the content providers and figure out a solution.
Clearly not all content would feasibly be captioned. No reasonable court would enforce captioning on "private" or indie content unless aired over broadcast/cable. The burden on captioning needs to be shifted to the content creators. There are already standards that determines what content must be captioned and anything below that is at their discretion. It won't be easy I admit, but this needs to happen, and the sooner, the better.