Because you can see the whole thing at once. By being able to see the whole thing, it can be easier to work with, and that's always nice. It's sort of like the reason that many C++ programmers still use printf-type statements: It's not typesafe (though gcc at least can check types on printf statements), and it's not pretty, but boy is it compact and usable.

If it's a simple enough regex, then they often ARE the best way to do things - their compact form and many capabilities make them perfect for certain tasks. The problem comes when you make the regex so complicated that you can't read it anymore, or when you're using a complex regex to do something that could be more quickly done via simple string operations.

Regex, like any other powerful tool, must be use in proper moderation - not too much, not too little. And unless performance is a big concern, a single regex may at times be quicker to write and easier to debug than a series of string operations.