Since when is it wrong to ask a hypothetical question?!
Because many of you have some concerns about my intentions, I will say here that I have no intentions other than understanding why these illegal sites are so popular and continue to exist now that sportbooks are legal in so many states.
Furthermore, I want to clarify the type of site which I am talking about.
There are these sites - ie) everysport.ag, bet.ag, parlaykingdom.ag ETC. - and these sites seem just to be a free service where anyone can become a bookie.  The sites provide the same service any of the major sportsbooks would provide. In many cases there are agents under a "backer" who is responsible for the finances required to pay people out. The agents get paid a certain percentage when their clients are net down.
Usually, these bookies spread their product by word of mouth and by offering current clients freeplay credits in exchange for refferals.
I am highly skeptical of anyone who chooses to use these books over a centralized alternative.

There are a plethora of illegal books, many run by college students, young adults and even teens. How secure are these websites??
What I am really asking is how easy (possible) would it be to hack the site and change a users balance without being detected??