How God fulfills his promise to Abraham, according to Paul, through Jesus Christ. He is the “offspring” which the original covenant promised. By faith in Christ one becomes an heir of Abraham, as witnessed by the activity of the Holy Spirit. Why would someone submit to the Law at this point since it is neither necessary nor beneficial?
My guess is that people want to submit to Law out of an honest desire to serve God correctly. What are the responsibilities of those who are now in Christ? This is an important question, and at least one answer to that question is to point to the already-existing body of commands found in the Torah.
Since the whole Law is not what the Galatians were doing, but rather the boundary markers, it is at least possible that the attraction was to define boundaries so that one could know who was “in” and who was “out.” Again, the boundary markers of Judaism worked well to define a separate people, so perhaps they wanted to adopt these boundary markers in order to demonstrate that they are “in Christ.”
Marking boundaries is very important to humans. Recall that when Jesus said that the second greatest commandment was to love one’s neighbor, he was asked exactly who was a neighbor. We want to know the limits – think of a child who is offered a cookie. The first question is usually “how many”? (a couple equals two, a few means three?) Take a kid to a store and they want to know “how much can I spend?”
But boundaries exclude as much as they include. Maybe people are attracted to legalism not for making themselves appear like insiders, but so that they can exclude people they do not like. Since you do not behave quite the way I define Christian behavior, you are “out” and I do not have to treat you like a brother in Christ anymore. Or worse: you are excluded for using the wrong Bible translation, or listening to the wrong type of music, or (gasp!) having a tattoo! Defining spirituality by external appearances is always foolish.
One of the real problems with Paul’s view of “freedom in Christ” is that we do not like to be free. We want the boundaries and rules, so we create more intense rules and regulations in order to separate ourselves out as spiritual. There is something comforting in a list of rules; I know my place if I am keeping up with the instruction manual. But that is not what we are called to, we are children of God, not scouts trying to earn another merit badge.
Paul would likely have a few choice words for modern Galatians!