Why would they want to do this? It could lead to more money and less exposure. It might not, but I think it is possible.
I am not saying that this will definitely happen, but I do think it is a very real possibility. If the Big10 and SEC become their own thing of P2, then a players' union would likely want to negotiate separately with them. If there are three levels, P2, subP2, and G5, then the union would likely want to negotiate separate deals with each group. Making the group larger would allow the negotiations to be with regard to the group, not the individual schools or smaller groups. It would allow the league to push for salary pool based on media revenue instead of based on total revenue. Another thing it could to is provide opportunities to areas/regions to be in contention. The NFL has proven that the model where every team has a chance at the beginning of the season raises interest and money for the group as a whole. The NFL schedule is based on previous year's records. When you play a team from another division, the top team in the division doesn't play the bottom team from another division. They play an equivalent team. Top teams play top teams. Bad teams play bad teams. Normal teams play normal teams. You end up with a lot of good games, instead of a lot of blowouts. It also means that almost every fan can look at their schedule at the beginning of the year and feel like their team has a chance. You don't get that excitement spread across the entire fan base in college football. It keeps people interested and excited even after their own team is eliminated.
I rambled a lot. As I said, I am not saying that this will happen. I am just responding to your question. There are some potential reasons that the Big10 and SEC might get involved in such a league. Potential to increase overall revenue, and potential to minimize obligations to players' unions.