Max exerting his influence I imagine. Can’t be many reasons.
You think Max is scared of a faster driver as a team-mate?
Well, he didn’t like being in the same team as Daniel and before then with Carlos, both of whom were much tougher teammates than Checo.
There are better drivers on the market. Plus Max and his dad have a lot of say in what goes on at RB.
Better than Max or better than Perez?
Maybe they’ve kept Perez ‘cos they won’t be keeping Max…Just sayin’ 
Ha ha.
With Perez keeping his seat at RB, that is one less possibility for Sainz.
Carlos will be racing in the Dakar 100% sure.
Does he have form in rallying or sand racing?
2+4
Seems odd that, given that Ferrari and McLaren will be pushing for the constructors next year you’d think they’d want two top drivers
Sainz wasn’t Verstappen’s teammate when he left RB (STR). With Ricciardo it was never personal AFAIK. Ricciardo wanted to leave because he felt the team was giving Verstappen preferential treatment.
Verstappen OTOH has publicly said he never wanted him to leave.
From what I know of Verstappen, I don’t think he would care who became his teammate. He thinks he would beat anyone currently on the grid, self doubt is not an issue he has.
Now watch Checo stay in F1 well into his 40’s delivering Alonso tier consistency. 
…delivering Bonzo tier consistency more like.
Sainz and Verstappen were team mates at Toro Rosso or whatever the team was called that week.
“Bonzo, my tyres are gone”
I suspect his dad may be able to give him the odd tip or two…
Correct, but then Verstappen got promoted to RedBull. Sainz was still driving for STR.