Plus, I think MGM in effect "rented" the prop/costume to various productions . And Trek, being particularly budget conscious, probably felt such funds could be better used elsewhere. (Of course, the way I'm phrasing my speculation gives the impression the production team actually considered it. I'm not stating they did. I'm just wondering how they might have handled the situation had it ever been addressed.
That's part of the reason (I suspect) Irwin Allen commissioned Bob Kinoshita (the creator of Robby) to design an "in house" prop, the rental fees if used throughout the series' run. As expensive as it was initially, in the long run, it was probably cheaper.
Sincerely,
Bill