I saw this on Reddit today, I can only imagine how these users felt when they saw 0 on their Coinbase accounts

Not Your Keys, Not Your Coins.
I would quote here this post by theymos, he perfectly formulated what the balance numbers on any service mean.
When you log into an exchange, yield platform, or other site, and it says that you have a balance of "1 BTC" or "1 USD", this does not actually mean that you have 1 BTC or 1 USD. Rather, it means that the company showing you this balance owes you that money. In other words, you've given a loan to the company. Maybe the company will pay you back, but history is littered with defunct companies which accepted deposits and were widely trusted for years, but which ended up not paying back their depositors in the end.