When was the last time that so many companies were making the same, or close variations on, the same basic design?
Domestically? World War II, probably, when you had fifteen different companies churning out each type of weapon.
Internationally, the FAL rifle was produced, in one variation or another, in Belgium, Argentina, Brazil, England, Australia, Canada (I think) New Zealand, (I think, briefly) Germany, and Austria.
Then, of course, you have the widespread production of the Kalashnikov, but most of this was done by state industries and not by "companies" per say.
Commercially, it's a pretty recent thing. It's because the patents have all expired on the M16 design. Anybody can make one, and sell it for profit without paying royalties. Furthermore, given that the US military uses this design, there's a very wide production base. Many companies don't actually MAKE their AR-15 clones, they just finish them and stamp their logos on them. The actual parts are made by various forging companies, and so forth.