AFAICS writing code for spesific features is (often) too risky for developers.
Most non-trivial features carry some uncertainty & chance of failure, which could be damaging for a developers reputation.
Anyone with the necessary experience to work on this needs to have the time available to devote to it, then have work to fall back on when the one off project ends.
Not that it’s impossible, in some limited situations it could work, just that I can see why many developers would avoid this.