Because you have to split the payout 2/4 extra ways. Right now, the average lump sum is $200 million a year. (It's actually a little higher than that, but let's just use an even number). You divide that by 10 schools, and it's $20 million per school. You divide that by 12 schools, and it's $16 million per school. You add in that $46 million increase, divide by 12, and now you get back to $20 million per school. There isn't a net increase. It just gets you back to the same payout you had with only 10 schools. The way you were phrasing it, you said the payouts to each school would go up $20-25 million a year. It doesn't. The actual payout to each school stays the same. The contract just goes up enough to cover the cost of splitting the pie extra ways, so the schools don't lose money.
Now, if you don't give the new schools a full share, then yeah, the original 10 schools will make more money. That's exactly what the Big 12 is planning to do. That's really a jury rigged way of getting more money, though.