I think part of this is the ego component: it's much more fun to make a relatively big contribution to something new than it is to fix a small part of something much larger.
That, plus many programmers are former nerds who just love tinkering with stuff and they invent languages as thought experiments. Which is 100% cool with me but it gets weird when they start to promote them...
And figuring out the gap between "wanting to tinker" and "being a reliable paid professional" is a big struggle that takes a while to figure out -- and maybe takes your entire life in maintaining a good balance between both.
