I got 2 pentas in ranked this season. They really that rare?

No

its just luck

most of the time you wont land all 5 last hits on the champ in the time frame

In higher elo ranked games they are. but ive once gotten 3 in 1 day as akali. I dont think its 1 in 1500 per player like riot says though

Yes. Numbers don't lie. Total numbers (according to Riot) indicate getting a penta 1 / 1500 games, and witnessing 1 / 150 games (the first number divided by 10 players per game).

Depending on the role you play most and the champions you play, your odds are either drastically higher or drastically lower. Of course AD's usually have the highest chance of penta's. Certain champs, especially Katarina, have much higher chances of a penta as well.

Note: You'll see a lot more pentakills in lower ELO games (both normal and ranked).

It's important to note that elo and champ really determine penta chance.

People like me who jungle probably won't get pentas *to this day, I think a Quadra was my highest*, but the people who play low CD assassins with high burst like Kat/Akali, champs with decent sustained damage like Vlad, carries in general, or Darius will tend to make up for the lack of pentas I get.

Did you play 3000 games on this season?