Breakdown of Зачем ты лезешь на скалу без каната?
ты
you (singular)
на
on, at / by (transport) / to (a place) / to (in answer to)
без
without
зачем
why, what for
Questions & Answers about Зачем ты лезешь на скалу без каната?
Why does this sentence use лезешь instead of лазишь?
Лезешь is a form of the unidirectional verb лезти. The speaker is asking about an action happening right now, where the person is climbing in one specific direction (up) onto one specific rock. If they were asking about a general hobby or habit (Why do you climb rocks without a rope?), they would use the multidirectional verb лазишь.
Why is the rock in the accusative case (на скалу) instead of prepositional (на скале)?
Because лезти is an active verb of motion. When you indicate the destination of a motion (where someone is climbing to), you use в or на plus the accusative case. На скале would mean the action is just happening statically on the rock, without the sense of moving toward a destination.
Could I use почему instead of зачем at the beginning of the sentence?
Yes, but the nuance changes slightly. Почему asks for the cause (Why?), while зачем asks for the purpose or goal (For what purpose?). Using зачем emphasizes the intentional (and perhaps foolish) nature of the action, essentially asking: What are you trying to achieve by doing this without a rope?
Why use канат for rope here instead of the more common word верёвка?