That just shows that English doesn't have an inflectional future, it doesn't show that it has no future tense. A tense being marked by a modal verb is not uncommon at all, German marks the future by means of the verb "werden", and French and Spanish also have an additional future (next to the inflectional one) using the auxiliary "to go" (e.g. "voy a comer").
The fact that future tense is not obligatory is also not an indication that it doesn't exist (again, it's the same situation as in German).
Compare the sentence pair:
"Every day at 6, I eat dinner"
"Every day at 6, I will eat dinner"
They have clearly different meanings, the second referring to something that will only start to happen in the future (which is why it's a bit hard to imagine a context in which the utterance makes sense). The first one refers to a habitual action that extends at least to the present moment.
The fact that future tense is not obligatory is also not an indication that it doesn't exist (again, it's the same situation as in German).
Compare the sentence pair:
"Every day at 6, I eat dinner"
"Every day at 6, I will eat dinner"
They have clearly different meanings, the second referring to something that will only start to happen in the future (which is why it's a bit hard to imagine a context in which the utterance makes sense). The first one refers to a habitual action that extends at least to the present moment.