I have opened over 300 regular cases (maybe even over 500?), but only received 1 gold and 0 cobalts from it…
They give you a 70% chance each time. That doesn’t mean that if you open 8 of them, you receive another card other than silver.
As RyePanda mentioned: If you flip a coin and you get head the first time, does NEVER guarantee you of a tail the next time.
In your case you are playing Bingo with 100 balls. 70 of them have an “S”, 15 of them have an “G”, 10 of the have an “W” and 5 of them have an “C”. You may only draw ONE ball a time and need to put the pulled ball back after receiving the card that belongs to the letter. Now: You drawn 10 times simply an “S”, meaning you had no luck that day.