Why do people send work related messages on holiday?

Early in my career I used to feel the same when my boss did it. On one candid dinner talk I asked him why does he ping on groups and DMs on work related stuff on holidays. His answer was simple - "I might forget the smaller things later in case I don't post it immediately, and hence I make it a point to post it as and when possible in case something strikes my mind" (he used to expect people to acknowledge and respond too, which was definitely a toxic trait, IMO). Today, while I lead a team, there are many instances where I post something that comes to my mind whenever possible (from late nights to weekends/holidays). I don't expect my team to respond, and have informed my team too that I don't expect them to, unless it's an emergency task to be done. I can quite relate to my ex-boss but I'm trying to avoid the mistakes he did.

So that people think you are working. Fucked up Indian culture. My EU manger used to frown if I send any mail on holidays. During appraisal, he made that as a negative point that am not efficient and needs to work on weekends.

Sounds like my manager.

Don’t respond to the mails.

I used to respond to such messages, not anymore. Definitely not healthy.

As to why some people do it - to let the world know what a toxic workaholic they are.