Because it is better for the sport. The SEC is pathetic for how they schedule. They get the media to artificially inflate their rankings pre-season, then knock one another off but stay ranked high "because reasons". Then they schedule cup cake FCS teams in November to ensure they don't drop. It's bad for the sport.
The time is coming when conference champions will automatically get into the playoff so the 9-game conference schedule won't hurt the champion.
Easy to make an argument that 8conf/4nonconf is
better for the sport: allows more opportunities for P5 vs P5 nonconf games, to mix things up more.
Also keep in mind we have the Alliance now. So each Big Ten team could be seeing two non-conf games per year against PAC and ACC opponents. That would necessitate going to 8 conf games per year, to then allow each Big Ten team to have 7 home games (last two non-conf being buy games).
I think that's perfectly good for the sport and a great thing frankly.
As to your comments about SEC rankings and scheduling - I don't disagree, but that doesn't really have anything to do with why they have to stay at 8 instead of going up to 9.
Problem for them is that four teams in each are locked into an intra-state non-conf battle (FSU Florda, GTech UGA, Clemson SC, Louisville KY), so that is a defacto ninth game for those four.
Then those tend to schedule cupcake FCS games the week before the rivalry matchup. Bama and Auburn do the same before the Iron Bowl, Miss schools do the same before the Egg Bowl, etc.