what without the T?

It’s replacing the [t] with a glottal stop. It’s not just American English that does this — syllable-final glottal replacement is common in a lot of dialects, and some British varieties notoriously replace [t] with a glottal stop between vowels under the same conditions that Americans flap [t] (“say it with a d”), like [ˈwoːʔə] *wo-ah* versus [ˈwɒɾɚ] *wadder* for “water”

>Why do they do this? why does anyone have any kind of accent at all?

We do pronounce it, either as an unreleased dental or as a glottal stop.

I always use the T. Maybe a regional thing.

From my observations, Americans seem to do this with Ts and some others at the end of stressed consonants.

They do this because: language.

You mean like this: https://youtu.be/6GEWSNylwS8?si=x3xZqKPN5xEb-sF9

"Why do they do this?" Because they are slow. Stupid. Pronounce the T, always. But avoid a "hard T", as in the word STOP, Soften the T in WHAT to be closer to a D. Not a D really, but closer to a D. As in BET, HAT, and CAT. Do not drop the T in WHAT, making it sound like WU. The word will be understood to be WHAT, but will absolutely give the impression that you are lazy and stupid. Dropping the T in WHAT is not an informal pronunciation of the word. Do not pronounce it that way.

In the case of "Wha?" specifically, it's likely due to something being so baffling that you can't even finish the word. Over time it just became normal so it's far more common. That's my guess anyway.

Like everything else in English, it depends on the person… some people actually overenunciate the T.

“Yaow, I done seen dat gyal dem out with anotha man.” “Whaaaa??”

That’s entirely regionally dependent. We definitely pronounce the T where I live.

This is what we do: https://en.m.wikipedia.org/wiki/No_audible_release As to why? Because we can.

Brevity.

As you learn the language more, you’ll realize that in speech, a lot of different things happen that you weren’t taught. Different regional pronunciations are one of them. Shortening words and phrases are another. For example some may say “withchu” instead of “with you”. These things happen to most languages when used by different people to varying degrees. You just need to be exposed to such things to get used to em. They’re not things that you memorize, but things that you learn and then use as you learn more.

It’s a glottal stop

americans for the most part pronounce T as D while thinking they pronounce it as T. Honestly the number of accents that pronounce T correctly is pretty small. In your example it's at the end of the word so even fewer actually pronounce it at all.

It's very common in American English for a t to become a glottal stop. I don't know why

A lot of the English accents have this too. Glottal stop, or simply dropping the T between words

Because making a D sound or glottal stop is easier than making a crisp T sound.

Lil John would like a word

You might be noticing linking which is common in an American accent. "What are you doing" becomes "Whadder yuh doing?"

So I guess you don’t consume much British media?

If the glottal stop T is a struggle for you, then dont come to the beautiful country of Barbados!

Usually it's to emphasize, surprise or disbelief.

Axe how americans say ASK?? Dont worry about American pronunciations, . it varies from region to region, culture to culture..

As Americans, we are lazy. This doesn't just relate to physical activity, but also how we speak. Most people just let words fall out of their mouth and make only the most minimal effort to correct enunciate.

Lazy tongues, but also because that’s how we roll.