They began to do that in earnest back in the 90’s. They were counting on collectors to buy and keep an increasing flood of issues, most of which never saw an envelope. They overdid it and killed the market. Stamp collecting has pretty much faded as it became impossible to keep up, and the stamps were printed in the billions so they had no collector value at that.
If you go to a “stamp and coin show” today you can generally buy US stamps at below face value.