Also, the "cheap plastic" thing is clearly said by someone who don't know plastic. NASA included a bunch of pieces on one of their Mars crafts to check the cameras since they knew the parameters of the LEGO pieces losing their color better than the cameras. LEGOs are durable.

Also, even if the unit cost of injection molding plastic might not be super high, the cost of making the molds is. I've heard discarded molds are used in the concrete for new buildings as a way to know where they are and ensure they're not suddenly in the wrong hands (although that might be a rumor)