XP itself only gives you levels. Levels give you War bonds. War bonds give you unlocks. You gain enough XP and Levels with it fast enough by normal gameplay that most probably with approximately 50 hours of gameplay you will have enough War bonds to unlock everything you have desired and then some. For this reason, gaining XP is not something people stress much on BF1 and hence, people do not really try specifically to fight over it.
If we implemented a system where high level people gained more rewards for XP, they would start farming it. And since the most effective way of gaining a lot of XP is to kill a bunch of people, we would basically get meat grinders in which high level people stomp on low level people in order to gain vast amounts of XP for their rewards. This would encourage further unbalanced teams as it is easiest and fastest to gain experience when you can simply walk over the enemies. It really would not end well.
XP grinding can happen in many ways. There are multiple ways to achieve that. A common way as I mentioned earlier is the meat grinder method. Letting the enemy keep a position that is poor to defend and then bombarding it with vehicles for easy kills and XP without ever capturing the flag. Another common way is to just hold a line and farm XP through healing, resupplying and resurrecting without trying to even push forward.
What I am basically saying that it is not a good idea to encourage people to farm XP for rewards rather than play the objective.
EDIT: I am basing my opinion largely on experience with other FPS games with similar mechanics. For example, on Planetside 2, you gain "Certs" based upon the amount of XP you gain. With "Certs" you purchase in-game things, similarly to how War Bonds work on BF1. In that game though, due to the game being free-to-play, the amount of Certs you need to unlock everything is vastly higher. On top of that, the game has a "Cert sink" sort of system with Implants which you can equip on a character to gain small benefits to the character and they use as a fuel a resource bought through Certs. All of this makes XP a resource everyone is hungry for even late in the game. As a result, it becomes quite common that bigger clans simply camp the spawns in the base and bombard the bases from afar with vehicles be it through air or land for free experience until people get tired and stop spawning in the base, resulting in nobody really having fun. Sometimes people do not even bother taking the base afterwards and rather wait till people come back there so that they can farm it once again.
