Because that's the nature of a heroic character.

A hero isn't necessarily someone who does the right thing, but they do the necessary thing, often in spite of the danger, to try and make sure the "good guys" win in the end.

In A New Hope, Obi-Wan (a Jedi master) ventures out of the Falcon aware that should Darth Vader be present on the Death Star, he would definitely become aware of him and would so endanger the escape plan and even his life.

Obi-Wan's real sacrifice doesn't start on the Death Star, it starts on Tatooine where he takes two droids the Empire is looking for and protects them so they can make it to the Rebellion. He can't just leave Luke (the whole reason he's there) alone while he does it either. He's not aiming to board the Death Star, let alone confront Darth Vader. Vader doesn't even know it's necessarily Obi-Wan when he's in the hangar (in other words, Vader felt Obi-Wan before the latter left the hangar). It's only when things escalate that Obi-Wan has to pull out his last and greatest trick, and draw Vader's attention long enough for Luke and company to escape. Remember, Obi-Wan knew he could Force Ghost. He doesn't lose that confrontation, it just didn't quite end up how he was planning.

In Return of the Jedi, Luke is almost a Jedi himself when he goes to forest moon of Endor. He has already had a Force conversation with Vader and Leia and knows that Vader can become aware of his presence. Knowing this he goes to Endor and realizes Vader would know that he was there. He didn't know that emperor already knew about that, so it's quite risky given the whole resistance plan could fail because of him.

Luke went knowing the risks, but he wasn't counting on

1. Vader being on his command ship (it was a safe bet Vader would be on the Death Star, further away)
2. Flying right in front of the bridge of said command ship

Their proximity is what gave Luke away. Furthermore, the Emperor knew full well about the Endor crew anyways. He gloats over having lead the Rebellion into a trap. Luke's presence made no difference in the ambush that awaited them.

In The Force Awakens, Han makes the mistake of going to the StarKiller base, well aware of how the Force works. There is a little possibility that he doesn't know about this particular nature of the Force given that Leia might have tried to have a Force conversation with Luke countless times after he disappeared. Also, quite a number of years have passed. Given that the base is the main weapon of the First Order against the galaxy, it's a fair possibility that Ben would be there and would definitely know if Han is on the base. That would ruin the already fragile plan, Ben would know that Han wouldn't come to the base without a plan to destroy it or something. Han probably wanted Ben to be there, but he was definitely not prepared for Ben to know that beforehand and tighten the security or whatever.

While it's debatable if Han went to confront Ben as a goal, Han had to do a more important job first (drop the shields, which he did). It's only when Ben presents him with an opportunity to confront his son (as his wife, Leia, begged him to) that he reaches out to Ben. Han dies trying to reach out to his estranged son. In some ways, Han wanted Ben to find him.

In all mentioned cases, there was not much of a choice

As you said yourself, Kenobi didn't plan to end up on Death Star, even warned Solo not to follow TIE fighter. Once on Death Star, Kenobi didn't detect Vader immediately, and he had to turn tractor beam off while others try to rescue Leia.

Luke on the other hand gambled everything on Anakin Skywalker. He was completely aware that Vader could detect him aboard the shuttle and that he endangers the mission. But subconsciously he was actually hoping for it. Unlike his Rebel comrades, he knew that Rebellion could not simply physically win by destroying the Death Star 2 and Emperor on it. Instead, Dark Side had to be defeated and Sith had to be destroyed. He felt that he needed his father to accomplish this and he also felt that his father would not destroy him. What would happen without him on board? Imperials were getting suspicious because the codes of the shuttle were old. Had not Luke been present on board Vader would probably order shuttle Tydirium to be searched.

Finally, in Force Awakens, we are led to believe that Han Solo is only pilot (and Millennium Falcon only ship) who could pull dangerous maneuver of coming out of hyperspace inside the shield. Thus, risk of being detected was always there, but it was necessary. Again, like Luke, Solo did hope he could turn Kylo Ren (he almost did), but this is not primary mission objective.