Because of many reasons.
First is that in almost all situations people are giving Likes only when they are forced to be able to get download link.
So, for many "newbies" it would be "Mission impossible" to collect Likes just to be able to download something they need.
And then they will be forced to protect (hide) their complete post just to collect Likes they needed, and so on.
In circle ...

All this would lead only to reducing number of members.

Some members did hide complete posts (without download links) just to get more Likes, but I don't see them on forum any more, for very long time.

Protecting download with forcing to give Like to get link(s) or other part of post is just enough.