Question [Q] Custom build containing Play-Store and F-Droid

On any of my phones and tablets I use Play-Store and F-Droid regardless of original or custom build. While many custom builds include the Play-Store I haven't found any which also includes F-Droid. Why this? Don't others use F-Droid as often as I do? IMO F-Droid would be a perfect match to custom builds. How can developers be encurraged to include F-Droid into their builds?