why do we say scorching hot while scorching already means very hot?

Scorching means extremely hot. So why do we say scorching hot? Isn't it redundant to bring hot after scorching?

Scorching is both adjective and adverb. If you use it before an adjective like hot it means you're using it as an adverb, which means very or extremely. As an adverb it means very hot. So it's not considered as redundant info.

Examples:

Here in Houston, it was a scorching 93° today.

It was a scorching hot day.

The scorching desert heat

You're right, scorching heat (or scorchingly hot) appears at first glance to be redundant. Also, it is something of a stormy petrel (thank you again, Alex): You can't say something is scorchingly cold or scorchingly tepid.

Good writers might find a sharper way to express the heat, probably avoiding a cliché into the bargain. Still, in conversations and informal writing this kind of adjective (or adverb) is used simply as an intensifier. You'll agree, I'm sure, that scorching heat is many degrees more uncomfortable than mild heat. Is it as hot as extreme heat? I'm not sure. But both extreme and scorching heat refer to weather that is pretty damned hot.

In any case, something that is scorched is heated so as "to become dried out and lifeless" [Webster's] — so it has a particular meaning after all. There are all kinds of heat that do not cause things to become dried out and lifeless.

Scorch, as a verb, means to burn slightly or to cause discoloration due to heat.

As an adjective, scorching can be used to emphasize that the following noun causes burns or discoloration. In that regard, "a scorching heat" implies that the heat causes burns or discoloration. It is roughly synonymous with burning.