Why isn't it standard practice to rush an opposing goalkeeper?

Went to an MLS game last night and I noticed that in most instances, when a keeper had the ball at his feet, the opposing team would just let him have all day. Occasionally they would send someone to pressure him, but usually not. To use an American-football term, there was no pass rush, so to speak.

Now, maybe 90% of the time, rushing a keeper would just mean him passing the ball to someone before the defender actually arrives. But in the 10% of cases where you could pressure him into a turnover or some blunder, surely the payoff would be immense, right? Pass-rushing the keeper into a blunder would most likely mean a goal for your team in a sport where goals are very scarce.

If nothing else, rushing an opposing keeper every time would at least increase his stress for a position that is already the most mentally taxing of any in the game.

The keeper can simply take the ball with his hands, the attacker has no chance. And then he stands there and has to run back, because he is off-side: his team mates cannot pass him the ball even if they quickly win it back. Not worth the effort, usually.
If the keeper cannot take the ball with his hands, because he received it from a defender (not a header, then he may), the defenders position themselves wide and are usually in a good position to avoid being pressed. Offensive formations sometimes still try, but it is taxing and hard to keep up for long. And when the ball can be passed through to midfield the now defending team has at least one player out of position. Maybe more, as pressing requires more than one player. If the pressing is not well coördinated the counter attack can be more dangerous than the pressing ever would have been. It is seldom a winning move, but sometimes it is done, very seldom it works. Then it lands on YouTube.

Generally it’s because playing a press is pretty taxing. And the keeper doesn’t usually put the ball down until his general area is clear, so you’d have quite a bit of running by your forwards to pressure the keeper.

They’d probably completely run out of steam by half time to actually make runs to score the ball when your team had possession

Not if it is a teammate back pass.

That is what I addressed in my second paragraph.

It’s not standard practice because it’s not worth it.

Very aggressively pressing is pretty in style right now. However, it’s not done all the time, but selectively. You want to press when the team can press in quickly and in unison, there’s a pretty good chance at winning the ball back quickly, and a turnover will give you the ball in a good position.

If you’re talking in general, only the third is really true for rushing a keeper. Usually the space is too much, the team is shifting defensive structure, and the keeper can play out of pressure pretty easily.

If there’s a bad back pass and the opposing team is already in good position to press you’ll usually see the keeper pressured though.

Not standard since it doesn’t often work. It’s a bit of extra energy both to run aggressively at the keeper and then back into an onside position. Keepers tend not to be much perturbed by the maneuver and usually have a plethora of defenders they can kick the ball to. Best used very selectively, a 10% success rate seems like a very high estimate.