Depending on what exactly ur saying about people "not capping" there can be a LOT of reasons as to why they are doing that.
If you have just won a team fight top and you cap while one or more people seemingly just wander off, it may not be that they think capping is unimportant, instead they may be trying to scout ahead to prevent someone from suddenly showing up and roflstopping your cappers (think: ez, kass, malphite with ult, alistar, vayne, talon, etc.). Your teammates going to the jungle to cut off anyone may be the only reason you were given those 6 caps and 3 neutralizes.
If you are talking about capping or neutralizing DURING a team fight, this is a bit of a trickier situation. If can be useful if you are a support or someone with the need to capitalize on on their CD rotation (like brand) to cap the flag while your team is fighting in order to help prevent turret damage, or to complete a cap to help protect teammates. However, that will make it 1 less person in the team fight, which can cause the loss of the team fight. If you are neutralizing and capping, but your team is constantly losing the team fight because they dont have your damage output/cc, it doesnt really matter because the enemy team will just cap it back after cleaning up your team.
Also, your notion of "no one defending" may not be accurate. People seem to think defending requires sitting on the turret and never leaving. With garrison on the enemy team, this can be a bad idea as it gives the enemy team the element of surprise where THEY are the ones who decide when a fight is coming.
Additionally, sitting on your point in clear view of the enemy team allows them to know EXACTLY where you are and what you are doing. If your teammates are in your jungle hiding in the brush above the buff, or right next to the speed boost, it can be a far more advantageous way to defend as it makes your team MIA, gives them greater flexibility to prevent/recover from bot ganks/buffs/backdooring.
It is even more true when there is a person "defending top" and refusing to leave the turret when a fight breaks out in the jungle as your team caught theirs trying to backdoor/get the buff/etc. Being stubborn and thinking "Im just gonna sit on my turret and defend!" is basically the same as saying "Im going to let my team die!". While you sit on the turret to defend, your 3 other teammates are dying, and after they kill them, do you really think you're going to prevent the 2-4 people from killing you and capping your point?
I'll give you a quick example of what I mean because I recall at least 3 games with you, one where you were Ashe bot, another where you were Ashe top, and a 3rd where you were Lux top.
Ashe bot: I destroyed you with twitch... somehow. No point in reviewing cuz this game had nothing to do with what you are saying in your post.
Ashe top: Your arrow made you marginally better for your team, allowing you to sit top and still have an impact in the fight. However, again your staying on the turret when we engaged your team as they came back from healing/tried to wander allowed us to gain the advantage. Late game you finally started to come off the turret and you began to do rather well.
Lux top: As Lux top, we constantly engaged your team in the jungle while you sat on top defending, after we cleaned up your team, we came and easily killed you and took your turret multiple times.
Capping and defending are fine, but dont think because you are sitting top while ur team roams or gets ganked in the jungle means you are better. Also, just because you are capping while your team scouts/protects/ganks doesnt mean that they are bad.
Dominion is extremely dynamic and people tend to stick to one mentality and believe theirs is best. Learn to be flexible and change your tactics as to what will be most useful in the given situation.