The best part of the traditional sales funnel was that salespeople could move prospects through at their own pace based on their needs to close a deal. Today, customers are in control of the sales process. A modern buyer spends more time upfront researching a purchase before they engage with a salesperson, reducing the value of sharing information.
As a software engineer myself, I can tell you that building funnels from an application standpoint takes massive amounts of work. There's a great deal of coding and integration that's required here. From email systems to landing page implementations to credit card processing APIs, and everything in between, so many platforms need to "talk," that it takes the bar too high for the average marketer. 
If you're wondering what a sales funnel is, simply imagine a real-world funnel. At the top of that funnel, some substance is poured in, which filters down towards one finite destination. In sales, something similar occurs. At the top, lots of visitors arrive who may enter your funnel. However, unlike the real-world funnel, not all who enter the sales funnel will reemerge out from the other end. 
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