Because he would get signed by another team. It's either take that and be able to go actually play for a team, or rot on a bench. Even if the Lakers went the trade Deng route, he likely gets on a ****** team and never sees the court. He takes the extend/stretch and he can go join Thibs and get minutes/extra money.
There is no way the Lakers dont know if Deng is going to do this or not. I'm sure it's been agreed to internally as the reports say the Lakers are no longer looking into trading Deng (because teams demand Ingram or Kuzma).
Thibs when he sees Luol Deng is a UFA next year.