The TSA certainly has a problem with retaining its employees. In fact, it has some of the highest rates of attrition in the federal government. There are a number of likely reasons for this.
First of all, pay rates in the TSA are comparatively low compared to similar jobs. Many TSA agents make less than $40,000 a year. Given that many of them need to live in major cities with large airports and ports, it is difficult to provide oneself with a decent standard of living on such a low income.
Unlike many other federal jobs, TSA employees receive few benefits and workplace rights, including Family and Medical Leave Act protections. In fact, TSA employees can be fired for simply missing days of work due to illness or the need to take care of family matters. Unlike most other parts of the federal government, TSA employees cannot appeal penalizations to the independent Merits Systems Protection Board. This creates an atmosphere which many TSA agents find overly punitive.
Promotion within the TSA is also difficult. Many TSA employees have found themself stuck at low-paying, entry-level positions for much longer than they expected. Many have left in frustration over this lack of opportunity for advancement. Furthermore, many who are hired on a part-time basis with hopes of going full-time find that this opportunity never comes.
There is also a perception from the public that the TSA serves more as "security theater" than as an actual front line against terrorism. Many TSA agents report harassment and lack of respect from the public that they are serving.
All of this has led to very low morale within the TSA, which is largely responsible for the high attrition rates within the agency over the last few years.