He's been playing yes, but he's playing the least compared to the other four linebackers in the rotation. There is also the incident when he left the field vs. Louisville instead of celebrating with him teammates.
Sneed has a role, but the staff doesn't trust him enough in certain situations. His versatility is lacking.
It's a simple scenario where he could want more playing time that ND just can't promise him.
Also, a dude like Sneed, would be a big "free-agent" in the portal. Someone would likely offer him some really good money.