Why online crypto wallets are not secure as cold wallets?
Anything that's connected to the internet has a risk to be hacked. When you are not connected to the internet there is no risk to be hacked. That's why cold wallets will always be safer than online wallets.
With online wallets/exchanges you have 2 risks: your funds can get hacked or there is a counterparty risk because the owner of the exchange could run away with your funds.
A wallet on your computer is more secure because there is no counterparty risk but you can still get hacked. Computers can for example get infected with malware.
The most secure way to store your cryptocurrency is offline. That's why the ledger nano s (https://amzn.to/2CI3xfR) is so popular. It's a small investment to be secure.
The only downside is that you can't trade easily with a hardware wallet. If you're a trader and make a lot of transactions then you need the convenience of an exchange. But to store your cryptocurrency for the longer term a hardware wallet is the best option.(Video:)
For sale here on Amazon: https://amzn.to/2CI3xfR