Is this true? If so, why?
Usually it's because the questions – as they are typically asked – are either too vague and opinion-based, or else polling for a myriad or resources and/or ideas.
For example, if someone asks:
I'm having trouble remembering English words. What's the best way to increase my vocabulary?
There are plenty of techniques that will do that – but whether or not a particular approach will be effective hinges largely on the individual, more so than the technique.
The Stack Exchange has traditionally frowned upon questions that are based on individual tastes, preferences, opinions, and styles of learning. This boilerplate language can be found in the help pages for every one of the SE communities:
Not all questions work well in our format. Avoid questions that are primarily opinion-based, or that are likely to generate discussion rather than answers.
(Emphasis in original)