Why are there so many wooden speakers?

I think our collective chains are being yanked by the OP, but the clever responses by folks far more witty than me have made this an interesting thread.  So far as I can see, MDF is an "engineered" wood product, but more immune to resonance issues.   Wood is wood, it seems to me, plywood of any composition included.  There are almost zero cost-effective alternatives so that is the simple answer to why wood is used in speakers.  

I had the oppty to hear a pair of Tannoy Westminster speakers this week, albeit clones, but employing the same birch plywood construction techniques as the real ones, with the addition of a wood-encased super tweeter, all powered by a very high-end tube-based front end.  Heck, if i had known that "wood" speakers were so tragically flawed, I would not have come to the conclusion that they were among the finest speakers my ears had ever had the joy of hearing.  

I did have a couple of pairs of Green Mountain Audio speakers, with enclosures made of a granite epoxy resin and they were excellent, but not intrinsically better than other wood speakers I have had, and brutally heavy.  I now have Spatial Audio OB speakers, which I think employ MDF baffles, and I have found them to be Holy Grail of speakers.

I had a pair of KHL Model 5 "wood" speakers a few years, recapped and re-wired, which were pretty sublime, with lots of balls in the LF driven by a tube amp.  I wish I had never sold them, a great "old school" sound, with a luscious mid-range response. Yet on my list of speakers to hear would be DeVore "wood" speakers, which garner tremendous praise and are the speakers of choice for many audio reviewers.  That said, I have heard many Harbeth speakers over the years that I found rather unmusical and drastically over-priced.   At any rate, it is difficult to imagine moving away from OB speakers for myself personally.    

Maybe you should just get planner type speakers and take the box out of the equation... 

Answer to O.P.: Because there are so many trees on earth. 

There's nothing inherently inferior about wood as a material for enclosures. There are certainly man-made materials with different damping/energy storage properties, but these often come with weight & cost issues associated. Moreover, there is no one man-made material I'm aware of that consistently makes a superior speaker enclosure, no matter who designs & makes the speaker.

Arguing the merits of different enclosure materials is somewhat like claiming that this or that digital chip makes a superior DAC: a helluva lot else goes into making a DAC, just as it does into making a loudspeaker.

My speaker cabinets are made out of doped paper and my drivers are made out of MDF.

@timlub, have you ever considered using tiles to enforce a box?  I'm just a hobbyist who likes tinkering and I wondered if including a ceramic or porcelain tile in the wall would make a big improvement without costing much since they're mass produced and available for low cost per unit.  Say 3 layers of 1/4" mdf where there's a big square cut out of the middle layer that is filled with a  super stiff ceramic floor tile.  It'd add some time to construction but not much to materials and I'd think it eliminate almost all box flex in the low frequencies.