Zeus learnt his lesson badly. The last time he offered god hood to Percy, the kid decline his offer with zero doubts and second thoughts in exchange for noble requests and insultingly mortal life. After Apollo's reinstatement, the God's received a prophecy that mention's the sea spawn, with the prophecy in hand, the God's decided to take the matters on their own hands, leaving them with no choice to forced the god hood on the powerful demigod.