If I want fast food, I go to a Wendy's. If I want a real nice steak, I go to the top tier steak house. Same goes for history subs. The community here likes those long, in depth posts and learning all kinds of new things and contexts. That doesn't happen in a one or two sentence tldr. There's plenty of other places that do that on the net. We try for something different here.

Because r/AskHistorians exists to create a platform where redditors and internet users in general can receive near academic-level answers to questions on historical topics, events, and phenomenons. Since such questions can rarely be answered in anything resembling a comprehensive manner with only a few sentences, the subreddit tends to generate long form (at least by the standards of the internet) answers.

History is rarely simple. When I reply to a question on AskHistorians, I do my best not to include information irrelevant to the topic at hand. Every word I write is written with the intention of helping you better understand the answer. With that in mind, implying that a tl;dr would somehow be a better use of your time is, frankly, a little bit insulting. Considering how easy it usually is to find short form, and often misleading, if not outright wrong, answers to questions on the internet - your question is a little bit akin to someone walking into the only vegetarian cafe in town, and then loudly complaining that they don't offer beef burgers.

TL;DR: If you can't be bothered to read a complete answer, you have most of the rest of the internet to cater to your tastes.

Because people come here for the depth an nuance experts can provide. They are here for the long answer.

Because if you're asking the question, you wouldn't want to read the tldr.