Why can’t you perform single displacement reactions with group ia metals?

Answer is: because alkaline metals (group IA metals) are the strongest reducing agents and most reactive metals.
Reducing agent is an element  or compound that loses an electron to another chemical species in a redox chemical reaction and they have been oxidized.
Alkaline metals tend to lose only one electron in redox reaction.

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2