The phrase "not your keys not your crypto" being parroted around to those without technical skills is to blame for many thefts imo.
Why has leaving coins on an exchange been villified? Perhaps it was Mt Gox type events.
In general however, coins on an exchange are protected by their server security, your 2FA (Binance is like 5FA these days), and importantly their insurance if anything was lost. KuCoin tested this when they lost coins and reimbursed all.
Storing coins on dodgy phone software wallets that have code bugs, are always connected to the internet, can easily be trojaned with a keylogger and other malware (in addition to the "normal" amount of spyware every phone has) that gives a lot of surface area for an attack on your keys is far less wise than leaving on the exchange imo.
Do it properly - hardware wallet kept in a hidden fireproof safe that not even your best mate knows about or keep it where it's insured...
