Why didn't the Soviet Union invade Pakistan in the 80s with India's help?

Russians are known for being unknown, unpredictable.
But before thinking on why not attack, I'm trying to understand why should they have attacked Pakistan.
We may have had good reasons to wish for a Russian charge into Pakistan but did the Russians feel the same way. Probably not.

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They would have eventually invaded. Russia entered Afghanistan because it wanted to reach the Indian ocean. Karachi may well have been a city of a Pakistani SSR . Alas! How much better the world could have been.

Pakistan Dirty, Deadly Game

The Soviet armored personnel carrier, loaded with infantrymen and flying a white flag, rolled up to the Pakistani frontier post of Tor Kham from the Afghanistan side of the border. It was the climactic moment of a battle that had begun after Afghanistan's mujahedin resistance fighters attacked and briefly held three Afghan border posts on the Khyber Pass. The Soviets had reacted with lightning speed, sending in a full brigade by air to retake the outposts. In the confusion of battle, three soldiers of the Soviet-backed Afghan army fled to Pakistan, but their defection had been detected.

A Soviet captain emerged from the personnel carrier. "We want the three men back," he said, addressing Pakistani frontier policemen in English. Beside | him, an Afghan officer repeated the request in Urdu, adding, "If we don't have them back, you will be in for a lot of trouble." The Soviet vehicle then turned around and rumbled back into Afghanistan. "Not a shot was fired," a Pakistani officer recalled. "But just in case we didn't believe they meant business, they dropped 80 artillery shells on our positions that night." For the next two days, sporadic tank and artillery fire fell on the Pakistani outpost--and on the morning of the third day, the Pakistanis sent the three deserters back. Says a Pakistani intelligence officer: "There are a lot of changes on the border. The Soviets are now much closer than they have ever been before."

Indeed, the sounds of bombing by Soviet MiGs and the crash of artillery have been growing louder and more frequent in recent weeks. Last year there were 81 incidents in which the Pakistanis claimed their territory was bombed or strafed by Soviet aircraft. So far this year 56 such violations have been registered, and in the past month there have been at least 60 artillery attacks as well. Soviet and Afghan government forces have also mounted several ground raids along the frontier, including one last month that involved several hundred Soviet tanks as well as fighter-bombers and helicopters. A few days later, Soviet infantry and helicopter gunships in pursuit of guerrillas attacked several Afghan border villages, killing more than 100 civilians.
The heightened activity has led Western intelligence sources to conclude that the Soviets are making a greater effort than ever before to destroy mujahedin units operating from sanctuaries in Pakistan and stem the flow of weapons and supplies provided to the resistance by the U.S., China and several Muslim states. The U.S. pipeline alone is delivering an estimated $250 million in covert aid this year. Additional humanitarian assistance is going to the 3 million Afghan refugees in Pakistan, whose number has increased by 500,000 over the past year.

Pakistani officials suggest that the situation along the frontier has worsened since President Mohammed Zia ul-Haq met last month in Moscow with Mikhail Gorbachev, the new Soviet leader, and Foreign Minister Andrei Gromyko. Zia was told by the Soviets that Pakistan's policy toward Afghanistan --collaboration with the resistance and cooperation with the U.S.--could cause the relationship between Moscow and Islamabad to deteriorate. Though that line was not new, Zia was said to have been shaken by the conversation.
Pakistan Dirty, Deadly Game - TIME