It is not stupid question, it is a good question.
The answer is that primes get more and more rarer, and there will always be large gaps where there are no primes:
I believe it is more efficient to examine a certain number that you suspect is a prime (looking like Mersenne numbers) and check if it is prime. Instead of checking many numbers in a row. There is no way to point out a prime number, no one knows if they are more frequent in some "areas" than other. So, we just basically check a random number - which takes a long time to do.