...at the cost to having your information and account hacked. I imagine some people still don't know how to clear their browsing history, and so will get caught by their spouse in that way.
There's also the problem of the messages you've been sending the person being printed out, or saved, which can be used as hard evidence against you to prove you're cheating.
In regards to hacking, to be honest, how many of these sites have been "hacked". And what does that even mean? No, take a second to think it through. Fine, for the sake of argument, lets say your information is in one of these sites. I say big deal.
Over a period of a few years, one of these cheating sites will amass MILLIONS of accounts. You will be protected via the simple principle that schools of fish have employed for millions of years. Through shear numbers, you will be protected. Through shear informational overload you will be protected. I mean, what is the hacker going to do, take the time to "personally" contact each and every phone number hoping to contact a spouse and tell a spouse? How about send everyone's spouse an email. One, there is the problem of finding the spouses email account and Two there is the problem of spam mail and the sheer volume of that so again, logistics protects you.
The only way you get caught over the internet by your spouse is if you are stupid enough to leave your password on your computer account. So sure, that is a possibility. However, in terms of internet cheating vs traditional cheating, internet cheating is far far safer. You can find the "exact" person you are looking for and do it in a way that ensures 100% anonymity. You can find someone who comes from a completely different circle, hell even someone who lives in a completely different city!!!!
Traditional cheating is very limited. Vast majority of the time you are cheating with someone from a social circle that either you and your spouse share or that your spouse is familiar with. Since that person (most often a coworker or colleague) is in the same circle as you, your spouse, and friends the probability of you being seen by someone else in your circle (while you are courting) is very high. Then there is the matter of the breakup. Traditionally the person you cheat with KNOWS your social circle and when you break up, sometimes that person seeks retribution and tells the spouse. Basically, with traditional cheating it is virtually impossible to isolate the person you are cheating with from your social circle. Whereas internet cheating does NOT have that problem. You can court anonymously over the internet and from day 1 isolate your inner circle from the person you are cheating with. In fact, oftentimes that person also wants to isolate you from their inner circle as well.
Not to endorse cheating mind you, but all things being equal, internet cheating is superior to traditional cheating. Think about it. It's not even in the same ball park.