Because then you also need to host a feed, and keep it updated when new releases become available, and keep it updated when new plugin releases become available, and ensure the feed stays up, and maintain patches, and maintain required dependencies. All the things that package maintainers (thank you!) do for the ecosystem. Unless there's substantial gain why not just stick with Nginx?

For what it's worth though FPM is awesome, and has made my life better a number of times. If you have to have software that isn't packaged and you aren't familiar with packaging, look into FPM.

https://github.com/jordansissel/fpm