A big part of it is that War Thunder is a game of operator nations, something that supports the fact that the majority of the modes are line-up-based.
You’ll find exported and captured vehicles have been added to War Thunder long before ground was a thing. It just wasn’t as common until two things happened:
March 10th 2021, aka update Ixwa Strike and its introduction of the sub-tree system. As nations are given a home, letting them use their stuff in a line-up together. Half the nations that are sub-tree are only because Gaijin deemed them to “not have enough” (well, this is debatable with many); it does show that sub-trees are as important as the nation that got the sub, as it’s a two-way system and not to be one is more important than the other. Even if sometimes Gaijin and users act like/want one nation to be treated as if it’s lesser than the other.
But also, people have been complaining for years about how 1/2-3/4s of the nations that used X in-game got it, so they ask, “What about Y nation?” (just ask nearly every Brit main and more than half will complain about missing “C&P”). So Gaijin gave the vehicle to everyone who used it because of those voices.
The exported vehicles give options for line-ups, as having too many with C&P is better than not enough with only unique (large part of trees are unplayable with 1 or 2 vehicles pure BR). And allows making thematic line-ups.
It’s also easier to model and add a service vehicle that sits in a museum than a prototype that was destroyed.