It takes more time/energy to read or write a longer and more thoughful stuff.

TLDR. Limited attention span.

Also when you actually think long enough about a reply, you might realize it's all less clear than you innitially thought and the points you wanted to make suddenly don't seem as valid or that you don't really know much about some part of the argument and just cancel the reply.

When you hit the character limit, you can always use a thread.

Short posts catering to biases and emotions have so many engagement advantages and little energy cost for both sides. Even if you don't want to read it, you already had. Unlike with this long post, that very few will read.