So, when you bench something you are moving, pushing on, an object, it has a weight that you are able to move.
When you "static stretch" you push something that is a weight that you could not move. It is heavier than what you might bench.
Why is static exercise, then, not better than benching?
Big Bill
Muscles have both a concentric and an eccentric phase of motion. To work a muscle completely it must go through both phases of contraction an elongation.
Static holds (not a stretch) in which you hold a weight for a set period and then slowly lower the same is virtually the same as performing a negative. Such can be and often is used to break a plateau.
Arthur Jones used negative lifts with Casey Viator during the Colorado Experiment.
Fitology
In your comparison, there are really THREE different methods.
You mention the first two:
1 - you are able to move the 'resistant' object (as in bench press). This is called 'concentric' training.
2 - you cannot to move the resistant object. This is called 'isometric' training.
3 - the 'resistance' moves, MORE strongly than you can. This means that you 'lose' the struggle, and the muscle gets LONGER while it contracts, instead of shorter. This is called 'eccentric' training.
You are correct in thinking that isometrics build strength more effectively than concentric moves do. That's why it used to be very popular in the 1940s & 1950s.
However, training effects are so 'specific' that your strength gains only apply IN THE POSITION used for training. It also increases blood pressure more severely than concentric training does, which can be dangerous.
Eccentric training is even more effective, and also became very popular, but was found to cause injury VERY easily.
?
Because you are not moving the weight.