The premise of the question isn't quite accurate. Hitler and other Nazi leaders didn't invade the Soviet Union purely to get oil. Nazi motivations for invading the USSR are too numerous to list here in detail. Territory (getting lebensraum for ethnic German farmers to live an idealized rural lifestyle), politics and race (fighting "Jewish-Bolshevism"), economics and resources (looting Soviet raw materials and industry), and many other things factored into the Nazi decision to invade in 1941.

As for why the Germans didn't invade the Middle East instead, think about how they would have gotten there.

1. Overland via Iran - this would still require the Germans to invade the USSR to get to that "starting point.'

2. Amphibiously by landing in the Levant (Syria, Lebanon, etc) - this would have required heavy use of the Italian Navy, since the Germans didn't have much of a fleet in the Med. It also eould habe required landing ships Italy and German really didn't have. Plus, the Royal Navy and thr RAF in Egypt and Palestine would have savaged the attack, which would have been at the limits of German air cover.

3. Land in North Africa, then invade via Egypt - similar issues as Option 2. Also, as history showed, taking Egypt was far easier said than done.

4. Overland via Turkey - this would have meant violating neutral Turkey by invading. Or getting the Turks to join the Axis cause. Invasion would have been a difficult, bloody business. And it's unclear if Turkey would have been willing to throw their lot in with the Germans again.

5. Amphibiously by sailing around Africa and landing directly in the Middle East - aahahahahahahahhahhahaha. No. For so many reasons (the Royal Navy, lack of ships, terrible logistics), no.