a total of 32 grand final appearances out of a possible 58 is 55%. in other words, half of the grand final places have been taken up by a third of the clubs. Keeping in mind that some of these clubs have had deliberate advantages designed to make them successful (either immediately or long term), and it can be reasonably argued that when gc/gws were introduced the draft was compromised to a point that it made it easier to stay at the top (because it was harder for worse teams to improve). These teams have certainly been above average but life always varies from the average, and it hasnt been the domination that it sounds.That's certainly one way to look at it, but let's look at it from another angle that further supports the OP's findings...
For arguments sake, we have averaged 16 teams in the competition since 1990, meaning the par number of Grand Final appearances for each of those teams is 3.625 (2 of 16 teams make the Grand Final each year over a 29 year period).
Therefore, for any given group of five teams, the expectation is that those five teams between them will make a total of 18.125 Grand Final appearances over that 29 year period.
Between Geelong, Hawthorn, Collingwood, Sydney and West Coast, there have been a total of 32 Grand Finals, nearly 14 more than what is statistically expected.
So more than half of the Grand Finals won, more than half of all Grand Final appearances, it's pretty compelling that these teams are by far the biggest 5 of the AFL era.