why do you think some people don't say thank you when you hold the door open for them?

ignorance, or for some offence that you'd think they need the door held for them.

I just think it's good manners nothing more, no intent to send a message. just polite and manners.

I always say loudly "you're welcome" to anyone not saying thanks.

Most do and appreciate it. I do it all the time and I can’t remember any that didn’t say thank you

I think it has to do with the sense of entiitlement people feel nowadays - "I'm special, so I DESERVE to be treated differently." I give them a loud, "You're welcome," too.

Entitlement.

Pig ignorance and entitlement, usually resulting in a loud 'youre welcome!' from me