Everyone's profile, as well as everyone's goals, are different. Some people collect cards for the thrill of it, others are SUB chasers, and others just want to get as much credit as possible. As long as they're not maxing it out and straining themselves/putting themselves in financial trouble, I don't see anything wrong with it.
As to the "secret," there is no secret. It's simply what one ends up qualifying for, as well as time.
Answer: And the secret is........There is no secret-
Just very meticulous & careful management with some good timing sprinkled in here and there. If one really is filled with surprise & fascination comb these forums and not focus on a single member.
Because there are some in these forums sporting on the order of 1 Million Plus in card account credits. 
Takes time but is attainable. Whether for hobby or ambition or perhaps just collecting, or for every category to them purpose.
High CLs are easier with higher income but many get them with over time and proven responsibility.
GL!
To state what may be obvious, MyFICO forums are a self-selected group that can be pretty different from the population at large. What you see here may or may not be common in the world.
Also, there are people here pushing limits and looking for gaps and opportunities that most people would overlook.
Add a bit of cognitive dissonance when "generally good advice" is ignored and there seems to be no consequence, eg getting approved for stuff with numerous inquiries, and you do end up with some situations described on this board that some people find hard to believe, but they are real ... at least, I have never seen anyone caught lying, don't see why anybody would lie, and I believe the accounts I read here.
I've only accumulated $99K in CLs so far, so I'm probably not a good example of what you are talking about. But, 24 months ago, February 2019, I had no credit cards and scores in the low 500s. Right now, I am gardening, but I think if I wanted to push, I could get to $150K in a week or two.
It is largely about choosing your battles and being willing to accept pain in other areas. Like, if I did get that $150K, it would push back when I could get US Bank or Chase cards, could trigger adverse action, and would push back when I'd want to apply for a mortgage.
Credit is complicated and multi-dimensional. You might compare it with taking a pill to help your health in one way, which ends up hurting it in another way. Whether it's good or bad depends what you focus on.
If you wanted to increase your CL, I'd suggest looking at lenders known for generous CLs. Sync Lowe's and Navy Federal are examples. You can find others on this board. Look at the cards in people's signatures and note which ones tend to be larger or smaller. There are patterns.
JMO. Good luck!
