Because he's the best player we've ever had in my lifetime.
The opt out after 6 is better than no opt out for Betts. Plus, he gets more money in the first 6 years, especially his arb years, so if he invests it well, he can make much more money than just the salary.
What we gain:
We have the best player in Red Sox history (since Teddy) for 12 more years.
We front load his money, so if he opts out, he might make more at age 32 and we don't have to worry about his post prime decline.
Front loading also makes it easier to trade him late in his career.[
Our Luxury tax hit is just $31M for the last 10 years, not $35M.
We make a ton of money off the marketing value Mookie brings to us.
I'm sure there are more pluses I've not listed.