Learn Polish: Dlaczego często krytykujesz siebie? - Why do you often criticize yourself?

The verb krytykować usually takes the short reflexive pronoun się. However, we use the full form siebie when we want to add emphasis. Here, it highlights that you are criticizing yourself, as opposed to criticizing someone else.

Which case is siebie in here?

It is in the accusative case, which is the standard case for direct objects. The verb krytykować (to criticize) requires the accusative. Note that the genitive and accusative forms of this pronoun are identical: both are siebie.

Could I just say Dlaczego często się krytykujesz?

Yes, that is grammatically correct and very common. The main difference is that using się removes the strong emphasis on 'yourself'. Also, remember that unlike siebie, the short pronoun się generally shouldn't be placed at the very end of a sentence.

Why is the imperfective verb krytykujesz used here instead of a perfective form?

The adverb często (often) tells us this is a repeated, habitual action. In Polish, habitual and repeating actions always require imperfective verbs.