Having a plan on the shelf for how to invade is different from actually intending to invade. Israel not only had a plan to preemptively attack Egypt - it was very actively training for this plan and then carried it out. In contrast, it's fairly clear now that Egypt did not plan to invade Israel in June 1967, and was not in any position to do so. Like you, I've also read Benny Morris' Righteous Victims (which only has a relatively short section on the lead-up to the 1967 war).

The two points I've been making here are:

1. The traditional narrative that Israel fought a defensive war in 1967 is highly questionable, at best. It does not seem that the Egyptians were planning to attack, the Israelis themselves had a major hand in the escalation leading up to the war, and the Israeli military was extremely confident that it would quickly defeat the Arabs (an assessment that the Americans shared) and wanted a war.

2. This narrative that Israel gained the West Bank and Gaza through a defensive war is then used to justify Israel's more than half-century military rule over the Palestinians. Even if the Israeli narrative were 100% correct, it would not justify imposing military rule over millions of people for decades, while slowly robbing those people of their land and subjecting them to regular violence and humiliation.