Hi - Welcome to NP.
Part of your question was
why they pay huge amount to domain traders ?
Many (some say all), of the valuable domain names are already taken, and many of those are for sale by Domain Traders.
Consider these related names, which one looks the most professional and valuable?
1. easywebdomains.net <- This was available and I like the word "easy", I am trying to preserve my capital during year one so I bought it from a registrar like Godaddy for a standard price. However it's not the .com so the name looks inferior to the .com
2. easywebdomains.com <-This is taken. Godaddy says they might be able to broker it for me for £48.99. Then there is the negotiated amount which could be expensive. However the Godaddy estimate is $1209 so the wholesale value is somewhere between $60-$120. I expect the seller would want a lot more than that so no deal for me as a one man enterprise, but if I was a large company it could be worth it.
3. easydomains.com <- Now we are talking! This is taken but if it were for sale on the open market godaddy estimates it at $7595, and I would say the name could command that kind of a price as I doubt if you would ever see that for sale at a wholesale price. Bear in mind Godaddy is just one estimate and people / bots can estimate wildly differently depending on the criteria and knowledge they apply.
4. domains.com <- Obviously taken and worth mega bucks because it is a single word domain, in a valuable industry, with the added benefit of (probably) a decent amount of type-in traffic (people who directly type in domains.com into their browser). Companies would be prepared to spend to get this name.
I hope that helps as to why domain names increase in value. It is at least part of the reason. The other main one being how much valuable traffic it already pulls in - but I don't want to go off topic.