Would an OPENBSD rig running OOSPFD or OBGPD be cheeper? YES. Would it likely be as fast? PROBABLY (if not faster). Would anybody know how to use it? PROBABLY NOT. While it could be learned.....time has a value. The exception is if you're starting from the ground up and have IT staff and procedures of size that could actually support 'non standard' gear. AKA if you get hit by a bus and you're the only guy in your shop -- how easy would it to be to get someone up to speed on your current gear? or do you need to fly in a specialist.
TL;DR: We use the gear because we can find people who know it easily without having to fly in a specialist.