>Just put together some stuff yourself, from scrap wood and stuff most people have lying around anyway
The intersection of HNers and DIYers is much smaller than you think. Also when you're paying $2500 for a 500ft^2 apartment in SF you don't tend to hang on to things like scrap wood.
I agree but the crossover between the tech demographic and the DIY demographic seems to mostly be with using software and electronics skills to have add additional functionality to things. A few people will get into hobby CNC and car tuning but people tend not to branch out into multiple expensive hobbies so if electronics/software hacking is your thing you're probably not going to go too deep into those.
Most people won't pick up woodworking skills until they own their own home and need to maintain it and only people with woodworking skills will have a pile of scrap wood around. Keeping scrap material in general around "because it might be useful" is not something the HN crowd is generally predisposed to doing.
Basically, we're mostly a bunch of office workers so expecting more than a couple people to whip up decorations out of whatever is lying around is kind of unrealistic since only a fraction of a fraction of a fraction of people here are going to be in a position to do that.
And before everyone comments about how they're personally an exception and they drag sheets of plywood home on the roof of their smart car I'm talking about the typical case here.
Instead of focusing on the plywood part, let's just get back to the point that "everyone can make DIY Halloween decorations", even if you live in a tiny appartement. Pumpkins, candles, white sheet tied to a string for ghosts, and then let your imagination run wild with scissors and kraft paper.
Prison labor+questionable environmental record+environmental cost of shipping adds up to a really bad idea.
Just put together some stuff yourself, from scrap wood and stuff most people have lying around anyway, instead of horrible styrofoam junk.