You probably see them as generic, throw-away items, right? A radiator vent is a radiator vent, and low price matters. Right?
It wasn’t always that way. There was a time when people dreamed about the radiator vents we use today. They dreamed because there’s never been a steam system that didn’t need to be vented, and in those days, there were no automatic air vents. Venting was a manual affair. You opened and closed a petcock on each radiator when you wanted heat.