It’s actually two different reasons. Years ago jackpots were annuitized over say 20 year with the first payment paid this year and 19 additional equal payments. More recently different state lotteries have allowed users to choose to get the full amount (pre-tax) in 20 equal payments or the present value (also pre-tax) right away.
I can show you the annuitization calculations but it’s kind of boring.
Regardless of the winners choice the winnings are heavily taxed.