Because he hasn't gotten his big bag yet. He was on a rookie deal and his current 2nd contract is only paying him $17 mil this season.
Excluding rookie contract guys he's one of the most underpaid players in the league.
And getting that big bag a year early would only matter in case of a career-ending injury or a major injury from which he wouldn't recover to be a max player again. That might be a major thing for someone who hasn't made his bag at all (career earnings <$10M or something).
Lauri's career earnings are $70M so far.
Some NBA players buy their mommas $5M mansions, have 12 cars, their wives have an expensive jewelry habit and they roll everywhere with an entourage 6 deep. Then, sure, you need to keep that money going. However, Lauri's most expensive hobby is disc golf, his parents live in his childhood home, his personal trainer is a dude that has a 200 sq ft gym in a basement and his wife cares about the environment and sustainability, not diamonds. The dude already has more money than he will ever spend.
Getting the max still matters of course. It's how these guys compete and measure each other. It's a sign of respect. And it gives you status. If you're a max player, then you get treated as a max player & star - by the media, by your team, by the fans.
But in terms of Lauri's priorities for his next contract, getting it a year early probably doesn't make the top10.
Plus with the salary cap going up every year, signing something a year early might not even be a good deal in terms of career earnings - depending on his career progress and the contract that comes after the next one.