Because obviously everyone who participates in a discussion has an account, so they don't feel this annoyance other than maybe when sharing a link to someone else.
Inertia. Twitter has basically no value, it's just a bad app and bad website. It was never actually "good" for information communication, but it was tolerable.
When it comes down to 99% of these "why are people using sucky thing?" questions, the answer is inertia. Never confuse popularity for quality, they're completely orthogonal.