But it's not 20%. It's 58% (using the average - also, the average happens to be the exact # for 2018 as well - there were 33 5-stars this year).
You would not get similar results throwing darts at a board, even if you only had 3, 4, and 5-stars on the board.
If you group all the 3, 4, 5s together you have a total of 1,677 players per year on average. Using random selection, 1.97% of the drafted players would be 5-stars. Even 20% is far better than that. This shows that 5-stars are a lot different than 3-stars in general (using random selection, 80.1% of the drafted players would be 3-stars). But, at the same time, it shows that having good coaches counts for something. The recruiting rankings will never be perfect, and there will always be players who exceed expectations, and others that don't meet them.