I would recommend you this book. It has a great and comprehensive explanation about the difference between が and は. I will just try to rephrase it in a concise way.
- sets the topic.
- puts emphasis on what follows it.
- gives the idea of distinction.
- sets the subject.
- puts emphasis on what is before it.
- answers an implicit who...?
My reason for asking this is because "ga" according to the video tutorial I'm watching is usually paired with verbs that mean to exist or verbs expressing desire.
Usually but not always, が here is just doing what it always does, although it has nothing to do with existence or desire.
Now to answer your question. The reason is that when anyone reads (or write/say) a sentence, they imply a context with it. "I really hate banks" has already a context assumed where it makes sense. Think about how you assume the sentence to be said in an affirmative manner and not ironically to mean the opposite.
Japanese people share these implied contexts and that sets the use of が of は in isolated example sentences.
(I know not about other beings but) as for humans, [humans/we/they] are strong.
(Who is suffering?) Humans are (the ones) suffering.
You can say 人間は苦しんでいる and it would imply what I marked for 人間は強い. It's not by any means ungrammatical. My best guess is that Japanese people tend to use が to mark things the like, desire, etc. and as a consequence when they think in an isolated manner about a subject experiencing something they intuitively choose が.
How they inductively acquire the language may also play a role here. I mean how frequently a certain phrase is said within a given context.
Anyhow, it is just the way it is, as long as you understand the meaning of は and が, you will acquire this implied contexts as you engage with Japanese content.