Why don t NFL defensive players line up outside the offensive linemen?

There's no rule to stop them from doing that, but that would leave the middle open for run plays. With no one to block, the offensive linemen could run ahead of the ball carrier and take out the linebackers and safeties.

No rules against it. The offense could just run sneaks and dives up the middle all day long.

Lot of QB sneaks up the middle if the D-line lined up in your theory.

No rule,

It is used often in the "Wide Nine" defense, which has been employed against the pass.

However as good as it is against the pass and wide running plays, it also allows blockers to get to the linebackers and defensive backs with a lot of momentum on inside running plays.

Because they want to tackle him or her

Because defenses are structured to account for the gaps of the offense. As the others have said, not accounting those gaps certainly makes the offense's job easier.

There is no rule that stops them from doing that, the reason why they don't is because it would open up the run game for the opposing offense.

Go right ahead as the offense then, just runs down the middle.

No rule, just common sense. It might be easy to run the ball forward with no D-lineman in the way.

They would be shredded by the run game. With a massive hole in the defense right where all the blockers are, the offense wouldn't need to pass