starfinder – Why do expert monsters get saves instead of reflexes?

Those are just reference suggestions.

The Expert Array, as you said, is the most suitable for furtive Characters, as envoys and agents.

Expert Arrangement: choose the arsenal of experts for expert enemies, such as stealth scouts or non-combatants, as traders or advisors. The expert matrix is ​​also used for agent, mechanical and operative class grafts. Expert NPCs benefit from a wide range of skills, which makes them competent in specialized tasks, such as theft or sabotage.

But then we see that the DJ can freely exchange the saving throws:

EAC, KAC and Shot Saving Bonuses: use the numbers listed for the EAC of your NPC; KAC; and Strength, Reflection and Will saving launch bonuses. If it would be more thematically appropriate to change the numbers of save shots, you can swap them (for example, giving a combatant a low Reflex save and a high Will save, for example).

Personally, I would leave the Expert Array with good reflections by default, but the developers probably thought that such monsters would be really difficult beat at most parties and that probably was not going to be fun for everyone. After all, Starfinder classes have a much of skills based on reflection.