Is a "do" missing in the third sentence?

it’s a common thing to leave out especially in certain dialects/ vernacular. “why you keep doing it” still makes perfect sense to a native speaker, even if they don’t use the structure themself

Yup, "Then why do you keep making it" is proper.

This is typical in AAVE (African American Vernacular English). Using it adds to the humour. What bugs me is the inconsistency, as the first sentence uses standard English. You would omit "have" but keep the past participle. "Anyone ever told you..."

Yes, you're correct. However, native speakers often omit the auxiliary verb in casual speech.

Yes

Yes in proper/written English In casual conversation some people will either say it, just make a quick d' sound, or omit it entirely, depends on your accent/speaking style

Certain dialects would drop the "do", but yes, proper English would focuses that it's supposed to be there.

Yes, however this construction is common

As others have said, omitting 'do' in this way is common in certain dialects, mainly AAVE. It's a little weird in this case where the other sentences aren't markedly AAVE, which makes me think that either it's actually meant to be standard English (and is therefore a mistake), or the maker of this meme is a speaker of AAVE (or some other dialect) who let it slip on the last sentence.

To be totally clear, this "do" is omitted in some Black American dialects. Idk why people aren't just saying that.

Yes. Though the sentence could technically also be phrased "Then why keep making it?"

The final sentence is not grammatically correct. To be clearly understood, it needs the word "do" added. However, as a joke - it can work without it. It's still grammatically incorrect, but it's funnier (because it's missing the word).

Yes, the third sentence should be "Then why DO you keep making it."

Yes. Certain varieties of English have the option to drop verbs like 'be' and 'do' in some situations where those words might be reasonably implied. For example, "she reading" or "why you drive like that". It's often associated with AAVE and dialects influenced by it, but is not exclusive to those.

Technically, yes, but informally it can be ommitted

It sounds normal to anyone who's watched Mob movies: it sounds like first or second generation Italian-American English. It doesn't jar because in English it's ok to say "d'you" as it's a stress-timed language and the d is lightly pronounced. If it was "why she keep making it?", to my ears, at least, it would be too odd.

In casual/familiar speech auxiliary verbs are often dropped. Usually in questions. "Why you gotta be be like that?" "Who we talking about?" "Where you going?" As a learner you probably don't want to try to follow that custom. Wait until you're familiar enough with everyday speech to know when you can get away with it, and who with.

You could also remove the "you" instead of adding a "do", either would be correct "Then why keep making it?"

Informal speech and memes can and will omit it. But, yes, it should be there.

r/unexpectedcosmere

"Do" is missing. But some English dialects may occasionally drop "do" in questions. I'm thinking particularly of African American Vernacular English. The joke is that Aragorn is using AAVE conventions, to make him sound more casual, less formal, and dropping his mask off respectability, because he normally speaks in an extremely formal way.

Do might be missing, or it might just be a regional slang.

Technically, yes, but people sometimes talk like that. It be what it do and it do what it be. Capisce?

The 'do you' is not missing, not accidentally. It has been omitted intentionally. This is a common conversational way of asking a question and completely appropriate for a Lego video game. The structure is 'why+bare infinitive' and is extremely common in conversation. It's called an elliptical question. - Why complain when you're not prepared to do anything about it? - Why wait until tomorrow? - Why not take a break? - Why not at least try? - Why go there? - Why you lie about it? - Why take the risk? - Why you insist on telling learners that natural spoken English is incorrect, I will never understand. 😉

it's probably slang

Depends on the dialect, in some dialects omitting that "do" is completely normal, however in standard English, yes, a do would be expected.

It’s so desperately missing that my brain fooled me into believing it was there,

Yes but u could g et rid of the ”you”

Then why do you keep making it? Then why keep making it?

Yes, but no. This is probably intentional since it can be omitted in informal speech in certain dialects. So it's technically incorrect, but some native speakers do speak like that