I noticed this too and it’s disconcerting. I started thinking that since I’m holding the door open on my own accord (as opposed to them asking me to hold it open), they feel that don’t have to thank me.
Having a bunch of younger siblings, I know quite well the relief of someone holding the door open for you with a carriage so I’m quick to volunteer… but unfortunately like you said it often goes unappreciated. Seems like a combination of anti-social Brooklyn-ness (like they’re scared to talk to someone who they don’t know) and an attitude of “bishvili nivra ha’olam.”