maybe they're having so many bitcoin wallets for the anonimity,so no one can trace how much the exact amount of bitcoin they owned
Simple security reasons.
Most bitcoin users have many wallets because they fear that if they lose their only wallet that contains all of your bitcoins, then all of your bitcoins are lost.
Besides, having multiple wallets is convenient as well. I use xapo online wallet for multiple, frequent small transactions and have hardware wallet for long-term investments.