Because 99% of the time when someone identifies as a front-end developer they are saying that they have not been a developer for very long.
So what were people who were already established developers pre-2010 doing? Working largely on the back-end. So now that they do a lot of front-end work too, they face the choice, do they brand themselves as a “front-end” or a “full-stack” developer? I certainly choose the latter.
Which leaves most people who embrace the “front-end” moniker as people who have been coding professionally for less than six years. So yeah…they’re less sure of themselves.
Disclaimer: I use 99% rhetorically. It certainly is an overwhelming majority but I haven’t done any data gathering on the subject. That being said, I certainly don’t claim that front-end development is easy. There’s a huge amount of churn. There aren’t many other areas of development where people have to actually stay on top of language specs.