The recommended cell count for ales is 0.75 to 1 million cells per milliliter per degree plato (ca. 4 gravity points.)
For 5 gallons (~18.9L) of typical strength beer of 1.048, that's about 12 plato, so you'll need
1 million x 18900 x 12 = 226.8 billion cells.
According to wyeast, the smackpack contains at least 100 billion cells, which is half of what is needed. Similarly, the white labs vials contain 75-150 billion cells.
On the White Labs site, they mention using a starter to achieve the pitching rates above:
Homebrewers who enjoy yeast culturing: If a starter is made from a
fresh vial, one vial can be added directly to a 2 liter starter, which
in 2 days will grow to approximately 240 billion cells, to achieve a
pitching rate in 5 gallons of of 1 million cells/ml/degree Plato (with
a 12 Plato beer).
So, why do they say a starter isn't needed? The producers are trying to strike a balance between cost (how much yeast you get) and convenience, and also reflect that some people like it to be easy, while others are aiming for the best they can get.
You can make good beer by direct pitching, although you'll typically get lower attenuation and more esters, unless you regulate the temperature. For many, that may be fine, and easily outweigh the inconvenience of making a starter. For others, getting the absolute best out of the yeast is the priority, and making a starter just adds to the fun of brewing.