Here's why:
* "Be" is a verb that requires a subject. In this case, the subject would be the person or people being welcomed.
* "Welcome" is an adjective, but it's being used as a verb here.
Here are some ways to correctly express the idea of welcoming someone:
* "Please feel welcome." (This is the most common and natural way to say it.)
* "You are welcome here."
* "We welcome you."
The best way to say it will depend on the context and who you are talking to.