They either make a better pitch or accepts they aren't getting an assistant. It's a luxury, not a necessity, and no one owes them their free time. 
You do it yourself. Study the things you want to be good at, and improve your abilities. You don't need to pay anyone else, but you also shouldn't expect free labor for person projects. There's a wonderful number of artists here willing to give feedback, advice, and general assistance to each other, all for free, but most of them have their own projects and can't just cater to the whims of random strangers asking for free art.
There are people willing to collaborate here, but it's going to take the right person, pitch, and project to make that happen.
Most of us are adults who literally need money to live, so that's one reason why. 