I still dont know WHY people dont use a normal wallet.
* Lack of commitment - some heard that it's not so safe to keep a wallet locally and hope that an online business may cover the loses when something bad happens; they don't want to care about security, they don't want to buy a HW either
* Lack of understanding - many don't understand the difference between a wallet and an account. They see bitcoin like the cash in banks.
* Shallow understanding and laziness - "I can access it from the internet anytime" "I can transfer for free to my friend's account (inside the same platform)".
Keep in mind that the majority of people having bitcoin (or other crypto!) don't read (or read from wrong sources), don't care, and if they visit this forum they might do it only to earn some pennies.
The biggest reason out of these three is probably the lack of understanding since most people don't understand the difference and essence of a trading platform and a wallet. They think if their money is kept in the platform and it is called a wallet, they will be okay even if something bad happens, while in reality, it doesn't work that way.
If an account in an exchange gets compromised, they would never compensate you since they will call it your mistake as it is your responsibility to safeguard your account and enable 2 Factor Authentication, etc.