Well, it they are undocumented and live for years in the U.S. they have to work to make a living, and this work is illegal and it is a crime.   Pathway to citizenship is a permanent resident card (green card first) obtained by sponsorship by work, family, marriage, etc.  It doesn't come on itself without any effort.  And on the top of it is checking if they committed any crime.   This is the way the things work.