The answer(s) to a question like this are pretty obvious and not unique to the oil industry:
It employs a lot people domestically; almost 900,000 directly, if the industry lobby can be believed. (I'm offering the industry figures here, because the official BLS data is extremely badly organized on this; the derrick operators or "rotary drill, oil and gas" operators for example are not considered to work in the oil extraction business but are "support" personnel somehow, and the workers in petroleum refining I wasn't able to locate in BLS data; they're probably aggregated in the chemical sector somewhere. If we add the oil workers categories I was able to find in BLS, we get about 500,000.)
The US is an oil producer and exporter; in fact the largest producer in the world as of 2018, and might become the largest exporter as well.
The United States became the world’s biggest oil producer in 2018, and over the next five years, the nation will take aim at becoming the top oil exporter, according to the International Energy Agency.
IEA forecasts U.S. exports of crude oil and petroleum products will nearly double, hitting about 9 million barrels per day by 2024. At that level, the U.S. will surpass Russia’s shipments and threaten to unseat Saudi Arabia, the current top exporter.
Whenever you have lots of people employed and/or exports are involved, you can expect subsidies...
If you read the official US rationale(s) instead, they center on national self-sufficiency in energy, like preserving national production capabilities despite price fluctuations on the global market(s).
Also, the figure you have advanced for the US subsidies ($27B) seem pretty large given what I've seen before:
There are fossil fuel subsidies on the consumer side as well (with the usual reason of providing social protection to those who can't easily afford heating oil etc.), but I'm not sure adding those would give the figure from your study.
From the actual ODI study, which seems the actual source of that "G7 scorecard" and $27B figure, I see the figure includes all fossil fuel subsidies, including consumer subsidies (as I suspected it would).
I will not try to argue like K Dog that this study is Chinese or Russian propaganda, but there is an obvious criticism to using this ODI study to single out the US: most other significant oil producers are not in G7. From the G20 countries data (previous table), Russia had higher (than the US) oil-production subsidies for the years considered in that study.