Why is recruitment often left until it's too late?

Why is recruitment often left until it's too late, increasing the risk of making a poor hiring decision? 
 
Organisations with mature salesforces have statistics available on what their annual churn rate of sales people is and when this is added to their anticipated growth for the year ahead, they know what they need to do for the next 12 months, assuming nothing changes.

Rather than waiting for some top performers to be tempted elsewhere and for other members of the salesforce to run out of time, why not adopt a proactive stance? Start the search process before you actually need to hire and give yourselves the luxury of choice? In this way you will be in proactive control of the process, rather than reactively hiring the best person from those presented to you in a short time window.