|laziness, impatience, and hubris|
They are free as in beer.
And like cheap beer, they are bland, offer no flavor to speak of, and pass through your system and are (hopefully) quickly ejected.
They are trivial to install -- no modules, no dependencies.
And the security they provide is just as trivial. Be careful what you wish for, you may get it.
They are easy to use, just paste in the form supplied.
Just sign here and your soul, I mean, job, I mean server is ours. Also, don't forget to place this convenient "Hack Me" sign between your shoulder blades.
They cover a good range of jobs designers want done.
So does a raw silk blanket, but that doesn't mean you can't see everything there is to expose.
They usually work as expected.
No, they work as invested. If you go for the quick and dirty solution, you should expect to see some stains in your access logs. There *are* secure alternatives available, if you're willing to invest some education, some <local currency units>, and some time into learning better ways to do things. Sure, MW's free...but so is advice. People worth listening to are rare. Ignore their advice at your peril.