Why do some people believe that humans are "bad at" generating random numbers/characters like this?
In short, it is more than a belief: there is strong evidence that humans are not good entropy sources. There is a test for this

Man vs. Machine. Or, why Man is not a Particularly Good Source of Entropy.

Try to win!
So we don't rely on whether generating a random number from the mind or random keyboard typings and mouse movements that seem like a monkey playing on the computer from outsiders. We rely on good entropy sources like the /dev/urandom. That kind of sources comes from good research.
Some researches on supporting this;

1972, W. A. Wagenaar, Generation of random sequences by human subjects: a critical survey of the literature

1997, Brugger, P., Variables that influence the generation of random sequences: An update. Perceptual and Motor Skills

2005, Persaud, N, Humans can consciously generate random number sequences: A possible test for artificial intelligence. Medical hypotheses, 65(2), 211-21

2012, Schulz et. al, Analysing Humanly Generated Random Number Sequences: A Pattern-Based Approach

2012, Williams and  Griffiths Why Are People Bad at Detecting Randomness? A Statistical Argument

2014, Towse et. al, Not all numbers are equal: preferences and biases among children and adults when generating random sequences



Other online tests;

not so random with the collected results.