English equivalent of Greek "Έλα ντε;"

I kind of use this phrase a lot when speaking Greek, and sometimes I feel the need to say it when speaking English as well but cannot think of a good candidate, as a non native English speaker, I also don't know if there is an English equivalent.

The phrase feels a bit slang-ish so I cannot think of a literal translation.

The usual usage of it is when you need to answer a question with a rhetorical question because you don't know the answer and you are in question as well. e.g.

A. The buses were on strike and I had to walk home.

B. Why didn't you call me? I could give you a ride.

A. Έλα ντε?

Free form translation could be something like "Yeah, why didn't I?"

"Beats me."

This is still very informal to slang, and not a rhetorical question, but it is quite common. The subject ('It') (vaguely referential: 'the matter under consideration') is usually deleted.

From CED:

[it] beats me

also what beats me [is ...] slang ​

said when you do not understand a situation or someone's behaviour:

It beats me how she got the job.

What beats me is why she stays with him.

and from Farlex Dictionary of Idioms:

beats me :

(it) beats me [slang]

A response when one does not know the answer to a question [possibly self-posed].

A: "How long has this milk been in the fridge?" B: "Beats me. Check the expiry date."

A: "When's Ali's birthday?" B: "Beats me!

It beats me – I have no idea how to get to the mall from here.

....................

A simple "Now why didn't I think of that?" is doubtless more idiomatic and can be used in formal as well as informal situations, but is so transparent that it doesn't really qualify as an answer on ELU.

I think I would translate έλα ντε as good question, defined by the online Collins dictionary as:

If you say 'Good question' in reply to a question, you mean that it is a difficult question to answer, or perhaps that you are embarrassed about the answer or do not know the answer.

When asked why you didn't do something, good question, like έλα ντε, can be used to convey that you accept that you should indeed have done whatever it is you didn't do and that you don't have any good reason to explain why you didn't do it.

While έλα ντε, is slightly more informal and Greek also has a direct translation of good question (καλή ερώτηση) which is used in a very similar way, as a native speaker of Greek and English, I would use good question as the English equivalent of έλα ντε.

One more:

You tell me.

Which has the benefit of coming close to the pragmatics of the Greek, which literally means "Come on, already!"

The "already" (the originally Turkish particle de) expresses exasperation about not knowing, which corresponds to the emphatic "you" in the English imperative. The "come!" imperative is metaphorically a request to the listener to help the speaker out (because they might have a better idea as to why than you do).

Your guess is as good as mine. (Cambridge Dictionary online)

British English:

informal ~ something you say when you do not know the answer to a question

American English:

I have no way of knowing exactly what happened or what will happen

A. The buses were on strike and I had to walk home.

B. Why didn't you call me? I could give you a ride.

A. Why didn't I call? I don't know, your guess is as good as mine.