How Does the Future Have the Infinitive?

In Spanish there is a dedicated future tense that is formed by adding something to the infinitive form of the verb. The noun must be marked for absolute, which is a little weird. Or it's actually nomative, but its functions as an absolute or a nomative, depending. So you have, yo cantare, tu cantaras, el cantara. And this is really weird when you look at it in terms of morphemes because you're trying to analyze it and you say, okay, exactly how does the future tense have the, this other infinite meaning? When clearly these are finite forms, these are not infinite forms of the verbs.