When the Mega Millions jackpot is advertised as $130M with a $88M lump sum, they only have $88M on hand to give you (using current jackpot numbers)
If you choose the lump sum, they just give you the cash they had on hand for the jackpot (withholding some for taxes of course).
If you take the annuity, they put that $88M that they have on hand into an investment vehicle where they are GUARANTEEING a rate of return that will pay you 30 annual payments totaling $130M. If they’re willing to guarantee those returns, I’m sure I could beat them. Plus, since the annuity payments increase by 5% each year (to ensure the winner can keep up with inflation), that first payment isn’t 1/30th of that $130M which would be $4.33M, it’s actually less than $2M (and then taxes are withheld on that).
I’m good with money, and have always been a good learner, so I have no doubt that I’d be better off with the lump sum. Studies have shown that most people would be better off with the annuity, however.