Looks like you don't even know how exchanges traditional crypto exchanges with order books work. Who told you they are the ones who sell crypto to buyers.
And crypto addresses are viewable by the public. If a coins is legit like Bitcoin, there is no room to create coins out of a thin air minus it getting noticed unless if of course you are using shit coins and shit exchanges where you will shamelessly get scammed.
I don't say all sellers are the exchanges. Don't get me wrong please. I know the basic of an exchange. I just wonder what if everyone decided to withdraw their coins from the exchanges to their own wallets? There are enough coins on the exchange's wallet? Or the whole wallets are just numbers? They urgently stop withdraws when all traders decide to take their coins to their own wallets because exchanges doesn't have enough coins. I said 'traders' because of that a real holder never sell coin on the exchange if it's not miner.