It’s supposed to be a gamble. Eggs were meant to be a large majority of their income, which is why they contained exclusives to begin with. However, that was then abused due to the ability to time eggs. They’re supposed to contain more not so good than good Arkadions as a gambling system in which you are drawn to spend more for a higher chance at winning said exclusives. All Arks in golden eggs are or evolve into 6+ star Arkadions. If you choose to spend real money and don’t get what you wish, then that’s bad luck. However, it’s much more likely you will not win, so you can’t get mad at the developers because you caved into gambling. It falls back on you for choosing to do so in the first place.
The system gives you about an approximately 6-15% chance to win one of the better prizes, not including the 500/1000 gold spots in the eggs, with the chance being higher in gold mission eggs, especially when fevered. You still have an overwhelming chance to not win the “prize” Arkadions or gold, however, so just because you didn’t win, you shouldn’t act surprised when the odds are against you no matter how you spin.