Problem with RNG's in games is they aren't actually random.
They are a pseudorandom algorithm. And while that has benefits, (set the correct seed value, and you can replay a 'random' sequence exactly. This is what makes stuff like minecraft and procedural content feasible), it also has major pitfalls.
The first is that, well, if you know what the seed is, you can predict the result.
The second is, these things are inherently NOT random. They are unpredictable, but not random.
Finally, and this is the killer, many of these algorithms exist that simply aren't correct. They aren't random in a statistical sense, they are clearly biased towards certain values.
A naive programmer using such a pseudo-random function may not know the properties of that particular function, and thus not actually know how close to true randomness it is.
They use it without even knowing if it meets the criteria for random output. (obviously it can never actually be random, by nature. But if you don't know the seed value, a good one will give results statistically close enough to a true random source to be considered 'random' for most purposes. A bad one however, will exhibit very obvious and very non-random properties to it. This is easy to spot if you graph the output results, but most people don't bother. The better the algorithm, the longer it takes for an obvious pattern to emerge. A bad one may have an obvious identifiable pattern in just a dozen, or maybe a few thousand results at best. A good one, can get into tens of thousands, to millions or billions of numbers before you start to spot a pattern.)
Problem is, a lot of coders don't really consider this. An especially bad case is people that blindly use the C/C++ library functions.
the 'rand()' function that ships with most C compilers is absolutely atrocious, and yet lots of people use it without thinking.
Still, even with a perfect RNG, there's still going to be some highly improbable situations that arise. If a game involves random elements, you're likely to hit severe frustration sooner or later...