# Why is the exponential distribution chosen to model service time in Queuing theory?

Why is the exponential distribution chosen to model service time in Queuing theory ?

$$\lambda(t) = \lim_{dt\rightarrow 0}{\frac{\Pr(t\le T < t+dt)}{dt \cdot (1-F(t))}} = \frac{f(t)}{1-F(t)}$$
For an exponential distribution $f(t) = b \exp(-b t)$ and $1-F(t)=\exp(-b t)$, therefore $\lambda(t)=b$, i.e., constant.