It's not about running the game, it's about having it open for modification. Patching a game in binary form is ridiculously hard, anything than simple bugfixes is mostly a no-go. With the source code available we can add entire features, port it to Windows 10, implement 4K HDR, textures, new models, new missions... the sky's the limit. Additionally, it's really hard to analyze the game from assembly opcodes with hardcoded data offsets. With C code either we can read what it does directly, or add instrumentation or debugging code to it to figure it out.
And personally, I prefer doing ./game, rather than having to install a bunch of stuff and see my system cluttered by hidden folders.
Ok but - and again I'm trying not to be negative, simply trying to understand - if you want a flight sim that works on Windows 10 and supports 4k HDR, wouldn't it be a lot easier to just build it from scratch, rather than trying to decompile a game from 1989 that came on 5¼ inch floppies? I mean, even if someone just _gave_ you the source code of the 1989 game it would be more work to port it to Windows or as a feature than it would be to just build your own 90s era air combat game in c++ without any baggage.
Is it simply about squeezing more out of a classic game we love?