Because most of the time something that you have to pay for, can also be gotten for free.
Unless you have an extra $150 laying around, I would not purchase traffic.
I would save the money.
Again its just my choice and suggestion, because there are so many automatic traffic generators and mailing lists that are free it is unbelievable.
You want to think about leveraging your time along with your income. For instance, Google Adwords can cost more than $500 if you start using their keywords and you might not even find the page.
However did you know that you can choose niche keywords on your own and put them into your listings and automatically start being found in google?
This is absolutely free too, but it takes longer. Than there are alternatives like ClixSense which you can use to generate results. What's most important here is that ClixSense can do something similar to Adwords, without even spending anything.
There are traffic generators out there which actually hand you 20,000 credits or visitors upon sign up. These are also free, and all you have to do is have your website approved.
But it is all up to someone's own decisions. The way I look at it however, is there are countless offerings out there.
All of them may be different but all of them offer something similar, so why spend any money at all or break the bank if you can get the same results over a longer period for free?
I tell many people nowadays they should save, and study. I believe all the ones who are really serious about doing business(online) will heed the advice or find their own version of it.