I assume there is the element of relatively rewarding quality over quantity with the 18 counters rule ( 16 for women ). Some people think they should go further.
18 is still quite a lot, but I guess you could get some Duracell Bunny players, playing just about every week and adding quite a lot more lower scores on top of their best 18. It is very questionable whether these extra tournaments and scores indicate that they are any better player.
I'm guessing it's for several reasons - ranking on a like by like basis rather than who can play the most tournaments. Protect players from over playing and getting injured and not to penalise players too much who have been injured.
While clearly at times may be a consideration, in the case of competing in USO qualifiers vs alternatives how "easier" an alternative may be for earning points wouldn't be near the top of my priorities.