Why doesn't HN has DM feature?

Ability to send direct message to HNers will be really helpful.

It does exist but in the form of opt-in. Anyone may put an email address in their bio like you have. Anyone that wishes to receive direct messages may put in a throw-away email address in their bio and I would strongly recommend using a throw away address. Other options could be listing Discord ID's or other chat platform ID's.

Some of the old school forums had a happy medium in that people could enable/disable DM's and/or emails per account. Something like that might be ok but if everyone could DM everyone it may cause some to leave or create addons and uBlock rules to disable that feature.

Not really. It would be an absolute nightmare to deal with spam and people would rather not visit the site than have it as a feature.

Those that are open to communication have already added their contacts in bio.

I find it strange you didn’t weigh this before asking the question.