"Stolen by hackers"
Yes but only if the owner of the coins computer system is hacked and/or They are careless
"It's at risk everytime it is used"
No, that's actually where it's incredibly secure. It's not a hack if some idiot sends BTC to the wrong address.
"They also recommend not keeping all of your bitcoin in one wallet"
Yes this is one of the best safeguards for personal mishandling of coins. Luckily these aren't real, physical wallets otherwise storing them would become a headache. For now we can safely encrypt address files into a tiny folder on a USB drive, an offline computer, somewhere in the cloud, etc.
Btw who is "they?" Seems like "they" spooked the shit out of you because it's really not that complicated.