For the same reason why it is in the present continuous in English. :) We accept the future tense here but the aorist would just sound strange.
For a lot of sentences with time expressions, you can use the present tense to mean the future, similar to English and many other languages. :)
Slight tangent, but I went to this forum which talks about the differences between preterite tense and aorist tense.
I'm not a linguist, but it seems that the general accepted difference is that aorist is a more specific version of preterite.
Preterite refers to imperfective or perfective
^^^ EXAMPLE: Imperfective = "I was walking"; Perfective = "I walked"
Aorist refers to just the perfective (so only "I walked" in the example above)
Yet someone in that same forum says there are times in Turkish where aorist does not refer to a past event.
I'd love to get the moderators' take on this to correct my understanding of the imperfective/perfective aspects, aorist in general, and also how aorist fits into the Turkish language.