Because for a very long time it was the case that imported oil from the region represented a huge % of petroleum consumed by the US starting in the early 70's, roughly 70% of the oil imported to the US in 1979 came from OPEC nations, and that year net imports were equal to about 40% of US total domestic consumption. Dependent is also a word that can be weighted differently, while its true the US had and does still have massive petroleum reserves in many forms, its not always accessible, or the market may not be interested in waiting for it if there are other options.

And it was not just say an economic move to get cheap oil vs using domestic supplies. There was a steep decline in oil production from traditional sources like say the Texas oil fields. And a lag in technology, knowledge, and capital for production in say Alaska and the Gulf of Mexico. All while generally domestic demand increased though this was not a linear process!

Between the challenges of tapping that next generation of oil sources, older price control measures, pushes for more fuel conscious industry and transportation, and regulatory limits, all were things that either were removed, or challenges to expanding domestic US production. While externally, price control efforts by OPEC and generally low prices for many years when production was stable made it hard to expand production.

The numbers are stark, between all influences and reasons US production went from a peak of about 9.6 Million barrels a day in 1970 to about 5.8 Million as recently as 2000 and the 20 year cutoff, though that number would continue to decline until 2008.

The rise of imported oil was also fast! In part because it was helped along by the lifting of limits, Ike for instance had approved a limit where imports were capped at about 10% of domestic production, this Mandatory Oil Import Program was in place largely until modified under Nixon in 1973 and most quotas lifted. And this is reflected by the import numbers to fuel US demand not being met by existing production. In 1973 its 3 Million barrels Net per day, a few years later its between 7 and 8 Million net per day, representing a jump from about 20% to 40% US domestic consumption. It then goes back down as a share for several years, worth noting too that for instance the Prudhoe Bay field in Alaska pumped its first barrel in 1977 and the completion of the Trans Alaska pipeline, and other measures like further removal of pricing regulations under Reagan for oil producers and the Iran-Iraq War. But the trend reversed itself again in the late 80's and would continue mostly unchanged through the 90's as US demand for oil continued to climb. Most notably Saudi Arabia almost on their own flooded production in the late 80's depressing the price per barrel and helping to capture market share and make life very difficult for producers the world over to compete.

So we have the fact that imported oil represented a huge, and at times roughly 50% of US consumption. This was for several reasons, political, technical, economic, and practical, but it was the state of affairs. And a very large % of that imported oil was sourced for the Middle East with Canadian production at the time facing similar challenges that the US did in Alaska(though for instance non OPEC imports when from about 30% in the late 70's to roughly 60% by the mid 80's! driven by our neighbors and a desire to diversify import sources). Now how that applies and what comes next in modern US policy and energy production with the huge changes of the past decade is beyond this sub.

We can also note the relevance of cultural memory, HOW we remember things is often dominant over how they really happened. Almost everyone, or their parents, or their grandparents has a memory of the OPEC embargo of 1973 following the Yom Kippur War and its effect on things like getting gas, or the scare in 1979-80 after the Iranian revolution. Those direct consequences as a result of events in the Middle East with regards to end user oil consumption can loom very large and shouldnt be underrated!

Some sources for the numbers quoted!

US Production by year: https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRFPUS2&f=A

US Consumption and Import numbers: https://www.eia.gov/totalenergy/data/annual/pdf/sec5_6.pdf

An interesting piece I came across written under the HW Bush administration talking about the issue in the context of that return to increased imports in the late 80's. https://www.eia.gov/totalenergy/data/annual/pdf/sec5_6.pdf

Another near piece I came across, from the New York Times in 1980, talking about the fall in imports for the previous year, and talking about the decline in production in the lower 48 but the continued expansion of production in Alaska. https://www.eia.gov/totalenergy/data/annual/pdf/sec5_6.pdf