Why is it safer to keep your bitcoins offline than online on Coinbase? I'm trying to think of a good way to store my bitcoins somewhere but I can't really think of a way to do it without

1) having something physical (eg. USB key) that can get lost or destroyed
2) having some sort of password (that I can forget)

The problems with either of those things is I'm planning on leaving my bitcoins to sit for a long time so by the time I need to sell them, I may not be able to remember all the details.

I was thinking maybe I should just leave my coins on Coinbase. At first that sounds more risky giving my coins for somebody else to manage, especially given what happened to Mt Gox. 

However, whilst cryptocurrency is kind of like the wild west, Coinbase is a legitimate company. They're also fully insured so if they do get hacked, it doesn't sound like I would be on the hook for any loss. In addition, accessing the coins requires two factor authentication and furthermore, they have my identification details. So these features protect me against me losing access to my own coins.

So please explain why it is safer to keep my own coins offline where I would have to manage and implement my own security instead of just keeping it on CoinBase where they have professionally implemented security, contingencies for if I lose my credentials and where my coins are fully insured?
Let's assume you keep your bitcoins on the exchange for 5 years. If the company behind the online wallet goes bankrupt, is hacked, or is malicious, you lose your bitcoins. If you kept them on a hardware wallet and kept the mnemonics in another, safe location, you can still recover them in case of fire, disaster, turmoil. 

Mt.Gox taught us that trusting in another's private keys for your holdings is not a good idea. Many of the people who lost hard in that episode are still waiting for a reimbursement.