Are you asking for some sort of intuitive explanation (beyond an answer just saying "That's how English grammar is"?)

Not necessarily intuitive, no, @Moshi

@msh210 Then just a regular explanation? It's that way because "when" refers to the time the action takes place. If you say "I'll eat when I'll be hungry", it means you will eat at the time when being hungry is still in the future (which doesn't really make sense). Taken another way, "present" is relative - in the future, you being hungry is in the present.

Taken even another way, "If you'll be around tomorrow" sounds like "If you plan to be around tomorrow" while "If you are around tomorrow" sounds like "If you happen to be around tomorrow".

English doesn't have a future tense, so what else could it possible do???

@curiousdannii English doesn't have an inflection for the future tense, but it can definitely express the future (using the auxiliary verb 'will'). If you read the question, you would see that that is what the asker is asking about - why not use the 'will' construction?