They do so because there must be a connection between QFT and ART.
Booth theories are well probed but we do still not have an idea how they are connected.
You can measure the effect gravitation has as an amount of energy and because of einstein energie is equivalent to a mass you can call it a (virtuell) particle and name it graviton.
The idea is that gravitation is somehow quanted like anything else.
This is an open question and the one who found the answer will get the nobel prize and come into the liga of einstein himself.
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