Why is activation enthalpy found and not just rate of reaction?
One of the Factors Influencing reaction rate is given below (Source)


  Temperature: Usually conducting a reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles, as explained by collision theory. However, the main reason that temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions (when bonds are formed between reactants). The influence of temperature is described by the Arrhenius equation. As a rule of thumb, reaction rates for many reactions double for every 10 degrees Celsius increase in temperature, though the effect of temperature may be very much larger or smaller than this.


Reaction rate , Activation energy are connected to temperature through Arrhenius equation. We are interested in Activation Energy as it gives an idea what temperature the reaction starts. We want to know $E_{a}$ in order to manipulate reaction rate essentially. Catalysts or Inhibitors work essentially on this principle.

Just looking at rate wont help us in varying the rates to our interest.