Because people will think: "If free is so good, his/her paid material must be 100x better! I'm buying this.".
Eben calls it: "Moving the free line."
So knowing that great product creators know this (usually mass marketed product launches) and they give away their BEST strategy in their free presell videos -- why bother then getting the product?!
Because you know you WON'T find 100x better information in their paid material.
For example the recent Frank video stated bad news gets higher open rates then good news. Well that's all one needs to know. Why get the entire course just to "hopefully" hear some more info about that strategy before it's implementation. Might as well discuss it on the forum.
Meaning: You can literally create a KILLER PRODUCT by extracting strategies from these free videos.
Solution: When doing presell videos with your BEST strategies, create a empty-fillers inside the discussed strategy. With phrases like "I'll show you exactly how to do this in the course".
In otherwords, give away the strategy but make parts of it incomplete, creating curiosity in the viewer, hence GIVING THEM A REASON TO BUY.
(This is different approach from giving away your best secrets and then saying "Get the course for more effective secrets like the one you just seen.")