Since the introductory word "that" in a predicative clause generally cannot be omitted, why is it omitted in the following sentence?

The answer is as follows:

The conjunction that can guide a noun clause (subject clause, object clause, predicative clause and appositive clause). It has no meaning in the noun clause and does not do anything. Element. When that introduces the subject clause, if it is placed at the beginning of the sentence, that cannot be omitted; if it is placed after the main clause, that is, when it is used as the formal subject, then that can be omitted. For example:

That he is ill is true = It is true (that) he is ill.

That can usually be omitted when introducing an object clause. But when there are words inserted between the predicate verb and the object clause or between that clauses, that cannot be omitted; if the object of the verb is more than two that clauses, the first that can be omitted, and the remaining that cannot be omitted; if the that clause If it is preceded by the formal object "it", then "that" cannot be omitted. For example:

He said (that) he was 45.

He told us, for the first time, that he was dismissed by the boss.

He told me that, if he was free, he would come to my help.

He said (that) he was 72 and that his wife was 68.

I feel it a pity that you missed the chance.

That usually cannot be omitted when introducing a predicative clause. But there is omission in informal English. In addition, when there is an action verb do in the previous subject clause, the that that introduces the predicative clause can be omitted. For example:

What we will do next is (that) we will have more people know the fact.

The truth is (that) he lied to all of us.

When that introduces an appositive clause, that cannot be omitted. For example:

We know the fact that he is fired.

The news came that he had won the prize.