I agree it's pretty unlikely, but... they would be completely ruined if they lost $120B in customer assets. There's no way they would ever be able to restore even a fraction of customer balances if that happened.
They are regulated, so before they were given a license, regulators required them to have a system in place and continuesly monitoring them to ensure that any entrusted bitcoins would be safe. Aside from crime insurance, which partially covers any lost BTC, they also have highly sophisticated security measures.
how they secure the bitcoins, Coinbase has strong security measures in place to protect crypto funds in users' wallets. Most notably, Coinbase stores 98% of user funds on cold storage. In this context, cold means offline. Funds stored on offline hardware are much more secure against cyber threats, such as hacking. Coinbase also uses 2-factor authentication (2FA) for logins and other actions within the site, which greatly reduces the likelihood of an account being compromised.
So only the 2% that's kept in hot wallets is more prone to hacking. The rest is stored in cold storage, making it much safer from cyberattacks.