Why do Thanos's punches not kill Captain America or at least cause some mortal injuries?

I don't think there is a good answer because you are pointing out a flaw in the writing of the films. Not a fatal flaw but one that irks true, long term fans of the Marvel characters. I think the real answer is that the writers and directors want to give everyone their moment and it is very hard to do that if you play by the "rules" of relative strength or power mentioned in the question.

Any villain that can defeat Hulk or Thor in physical combat would kill Captain America, Spiderman and even Iron Man. And it would happen pretty quickly. Captain America can be killed by a bullet... and was in the comic world, lol. Hulk and Thor laugh off bullets. So, yes, there are huge inconsistencies between "power" levels in the MCU and also at times in the comics.

The idea that Cap had Thor's power when he was holding the hammer might be ok given prior stories. Certainly in the comics that has been true--- even if it is often a poor plot device--- it, along with the human alter ego of Don Blake, helped Marvel reign in a strong character. It was Thor's kryptonite in many ways. In the MCU, Odin says this in the first Thor movie. The problem is, the third Thor movie kind of gets rid of that. Odin says to Thor, "what are you, the god of hammers?" And the answer is no, the power has always been in him... not the hammer.

So unfortunately, as good as the MCU has been, there are inconsistencies between movies. 22 films I guess it was bound to happen. If they thought these relative power things through, or wanted to, they could have been consistent. But they are after big moments and entertaining the general audience. The general audience does not care about this stuff. Half of them would be ok if an unarmed Pepper Potts was able to trip Thanos with a kick, lol, as long as it looked cool or was funny.

One more thought in response to other's comments. The Russos specifically stated in interviews after Infinity War that Thanos was not using the Power Stone when he trashed Hulk. So keep that in mind as you consider all the scenes with him fighting people like Spiderman or Cap. And that level of strength pretty much lines up with the Thanos in the comics.

If you were a general at war you would not send these less powerful characters against someone like Thanos directly. They might choose to do it anyway, but they would be hurt badly or killed. The shame of the stories is that none of these people really do get injured in these battles. Very little consequences for their heroics. I'd like to see Spiderman with a broken arm and ribs after slowing down Thanos for a minute. But no, he just gets right back up.

The Russo Brothers said that Thanos did not intentionally kill anyone fighting him with 2 exceptions. Heimdal... because he was angry for sending Hulk away. Also Loki. One he has failed Thanos in The Avengers and then swore undying loyalty before attempting to stab Thanos. Not once did he use the stones to kill. He was going to light up Tony Stark with all 4 stones before Doctor Strange saved him. Russo's admitted as much. He only killed with the stones with the snap. No guilt involved. He just wanted fair and random. Stark just pissed him off.

If you think about it, between Gamora's death and the snap, Thanos doesn't kill anyone. He wanted the snap to take the lives of everyone so that he had less guilt. He was probably holding back against everyone since he didn't kill war machine or black widow despite having the chance

In The Incredible Hulk Blonsky is given an incomplete form of the super serum that Captain America took, with it he takes a full body kick from the Hulk that sends him flying into a tree and mangles his body. He's up and about a short time later. Cap with the full serum is more resistant, even if he was hurt his healing factor would mean he wasn't down long.

In addition to the effects of the super-serum that gave him his abilities, at the time in question Cap is holding Mjolnir. It is well established in the comics and MCU that the wielder of Mjolnir gains the power of Thor.

Whosoever holds this hammer, if he be worthy, shall possess the power of Thor.

_{Thor, The enchantment placed on Mjolnir.}

Asgardians are continually shown to be extremely resilient to physical damage, Thor more so than most. Given the stacking and interacting of powers and abilities as well as Steve's own force of will it's no surprise he was unaffected. Imagine punching a hellbent god on super steroids.

The super soldier serum that was used on Steve doesn't just make him stronger and more resilient but also gives him regenerative powers.

Steve Rogers: Dr. Erskine said that... the serum wouldn’t just affect my muscles, it would affect my cells. Create a protective system of regeneration and healing. Which means um... I can’t get drunk. Did you know that?

_{Captain America: The First Avenger}

On top of that Steve seems to be very badly wounded in both fights against Thanos. In Avengers: Infinity War Thanos actually seems to knock Steve unconscious for a bit.

In Avengers: Endgame Steve, and Tony/Thor, are all really hurt by Thanos but Steve carries on with the fight even though he is hurting. It isn't the case that he isn't hurt as bad as Hulk, it is that when he gets hurt he carries on. This has always been the case since before he was even given the serum and this is what makes him worthy.

Also note that Steve has survived falls from great heights before throughout the films, not always landing on his shield, so it is safe to assume he is more durable than you are giving him credit for.