Because all exchange platform need to the personal info.
Also I can't buy huge amount bitcoin and altcoins via the platforms.
Not all exchange required to pass KYC or you need to input your personal information. You can still buy bitcoin and any altcoin without submitting any personal informantion because some exchange only required it if you gonna withdraw a huge amount of bitcoin like 100 bitcoin. But, in your case you already stated that you can't buy huge amount of bitcoin and altcoins so it's better to use an exchange than buying it to a certain person which is more risky than using an exchange.
Well you still have a choice though. Several exchanges don't require kyc. It's just a bit riskier since these are lesser known exchanges that haven't built their reputation yet. Though it's not all the time that these exchanges will just take your money. Some are legitimate even if you're not required to submit your information. Again it's a risk you can take if you don't want kyc.