Ignoring for the moment the specifics of video game music, one reason to acquire soundtracks over streaming them is for the control and privacy it enables. Your audio files can be played when there is no expectation of internet access. Additionally, there is no possibility of your listening habits being catalogued and used in ways that you might not like (or even know about).

Back to the context of video game music, for the longest time it was actually difficult to find anywhere that would even sell video game soundtracks. Places like Bandcamp have since alleviated the problem, but I can speak from personal experience that long ago one used to have to hunt for downloads, often without care as to who was providing the DL. This kind of experience can leave a lasting impression that game OSTs are something worth holding onto as local files.

I use Tidal to stream my music. At the moment, I couldn't live without it. The algorithm learned my music tastes (which are fairly obscure), and makes really relevant playmixes/playlists for me, introducing many new artists I would never have heard about if it weren't for Tidal.

In addition, a greater share of the fees go to the artists on Tidal than other services. It's still not perfect, but it's better.

So those were more the reasons of "why I personally use music streaming services instead of downloading FLACs that are bundled with their product (for example, games)".

Why you wouldn't want to use streaming services? Like others here mentioned already, one could list here all the reasons for any streaming vs. offline services and goods. Little need to repeat them. I would just point out, that for the use case of using offline media when network or data connection is unavailable, streaming services allow you to pre-download content for those long flights or trips.

So, for me, the benefits of streaming outweight the negatives. Sorry OP, I approached your question the wrong way around, lol!