Kurt Cobain was clearly a very depressing guy and was constantly negative and complaining about everything. Even just listening to any of his songs makes this obvious, even if you don't know much about him. (I say this as a fan of him.)
Marilyn Monroe was less outwardly unhappy, to be fair, but if you know anything about her personal life, she had a very sad life. She never knew her dad, her mom and grandma had mental issues (and she was known to have her own), she was an orphan growing up, she had multiple miscarriages and divorces, she was a drug addict, she was treatedly badly by men and the industry, etc.
So it honestly does not surprise me that either of them killed themselves unfortunately. They were rich and famous, yeah, but that alone doesn't necessarily make someone happy.
I don't understand why people's first instinct is to always assume that they must've gotten murdered and that there was a conspiracy to cover it up (and that they definitely wouldn't have had any reason to kill themselves) when they obviously had a lot going on. This doesn't just apply to Kurt or Marilyn specifically, but other celebrities as well. They were just two of the most obvious examples I thought of.