Because that would limit monster design severely, as many monster types would have to be designed with the "hey we need to consider what will happen if a Druid gets this ability" caveat.
This. Wild Shape doesn't give you all the abilities of the thing you turn into because the designers want to be able to make, for example, a CR 13 Large size plant with special abilities that would be overpowered if given to a 10th-level druid.
It's still a fantastic ability. Being able to turn into a huge water elemental with +8 Con, DR 5/-, and immunity to bleed, sneak attack and crits has literally saved my druid's life. At least twice.
And that's not counting the time I helped overthrow an evil regime by turning into a sacred white raven...