It's definitely the ego/vanity thing for those Master-tier players who behave like this, but for Pro players, it's far more: it's their desire to attain the Master tier as fast as possible, disregarding one's true playing skills. The Master tier gives you various perks, such as Master-only equipment, participation in prestigious Master-only tourneys, etc. The fastest way to get to the Master tier, of course, is to quit every bad round you have, and only record the good scores. There are no safeguards on WGT.com against proceeding like this all the time, so that a player's average score here on WGT.com is next to meaningless.
For half a year now, I've been participating in the League Tables tournament, and what I love about this tourney is that it requires everyone to post real scores only, and there's no way to quit a bad round and not record the bad score. Suddenly, you see how human all the players are: Masters or Pros. It's not uncommon, in that tourney, to see Masters with low 60s average scores post rounds in the high 30s or even low 40s. If this were also required for every round here on WGT.com, not only in an off-site monthly tournament of 5 matches, the players' average scores on this site would be radically different (and by that I mean a lot higher), and the number of players in the Master tier would be significantly reduced (cut by half or more, very likely).