Why do the Sparkfun and Adafruit Eagle libraries use smaller footprints than the manufacturer's recommended land pattern?

There are probably different ways to answer this, but my opinion: most Sparkfun and Adafruit customers are probably hobbyists and small manufacturers, who will not necessarily have access to high-volume soldering process. They will probably solder by hand, or use inexpensive automatic soldering systems. So they need more spacing between pads to prevent solder bridges, and make cleaning flux easier.

TSSOP-28 package is manufactured by many companies, and so many datasheets exist involving this package. I am sure if you will take a dozen of datasheets detailing footprint for this type of package, you will have at least two versions of pad sizes.

In your particular case I think someone, creating devices for these chips, took TSSOP-28 package from ref-packages library and applied to the device without comparing to particular datasheet.

I had a case when I designed footprint for the chip as manufacturer advised, and soldering chip by hand was simply nightmare - because pads were just short for proper access by soldering iron to put solder onto.

Thus if manufacturer recommends longer pads I would follow its advice. Regarding width of pads - up to you, depends on hand soldering skill you have and flux you use.