Philly has to put their eggs in the Embiid basket. They made (I think) the right move choosing him over Parker (?), Exum (definitely), Gordon, Smart or Randle. But he is feast or famine.
Despite the upside of both Fultz and Simmons, without Embiid the Sixers are lottery-fodder for another 3 years. With Embiid healthy, they are challenging for a playoff berth this year and growing into a contender in 3-5 years. The extension is a gamble they have to take -- needed to show teammates and fans that they are finally on the road to winning -- and are willing to invest in the long haul.