it's not just a seal.. Quit second guessing the engineers and reassemble your hub with a dab of light hub grease... They're listed as a set because if one is bad, both pieces need replaced. the Dust cover requires Rigidity.. the Seal requires flexibility... Plastic of the nature of that dust cover is rather oily... the two can not be made as "one piece."
Now.. COMPRESS the seal as it will be once the Sprocket/snap ring are installed....... what happens? i bet it SEALS against the vertical surfaces it mates to, and possibly the outer diameter of another hub piece......
the dust cover is then allowed to float somewhat, reducing friction.
just a guess.
other parts i've seen sold as pairs... thrust washers and bearings... points and condensors... oil filters and sealing rings... banjo bolts and sealing washers... fuel filters and those annoying little squeeze bands... wrist pins and tiny circlips that can fly farther than most airliners..... etc.