It likely is like this on purpose, that is, some animals like the big cats and hyenas are far more rare than others. It’s easy to find African buffalos and fairly easy to find zebra, giraffe, even mooses and bears in other regions.
But I agree that it should not be like this. Looking in the forums I can see lots of people complaining they aren’t getting their albums filled in what I would call a resonable time span. (And no, spending/wasting 100+ flight hours until, by chance, you see a single lion or hyena, is not adequate.)
Achievements like this should be something that requires skill and maybe a bit of luck too but it should not be a lottery where finding, lets say, a hyena with a baby is as rare as hitting the jackpot.
Currently these albums are off-putting if your goal ist to get all the stars.
BTW. This topic is essentially a duplicate of that one:
World Photographer > Fauna, extremely rare animals spawn, can’t be completed.
You might want to vote for the other one which has more votes.