Centralized Crypto Exchanges have many, many overarching laws that they must abide by.  On top of that, in the US, there are many, many individual state laws.  You would be breaking the law as soon as you open.  Without an entity, you would be leaving yourself open to personal legal liability.

As for security ... do you have any idea how easy it is for someone to initiate a DOS attack on you?  Simple buffer overflow -> elevation of privilege.  And do you really want to be the next hacked exchange?