It's horrible, but ..... I suppose every woman has to learn that not all men, in fact the minority of men, are looking for a relationship
If she starts with the assumption that they are, it's going to be very disappointing and maybe upsetting.
If she starts with the assumption that they may well not be .... It's a better viewpoint from which to deal with whatever happens
I'm inclined to think these guys are similar to the older (right up to quite old!) men looking for sex and casual sexual relationships on e.g. OLD. They project their agenda (sex or at most fwb type stuff) onto women ...they believe women might want that too (most don't but ..),.
If thet get interest and indulgence from a woman, they get very excited and happy and carried away with the prospect that this attractive women is "up for it" with them. That's the point at which they often shock and severely disappoint the woman with dick pics and or overtly sexual comments .... when their thinking and agenda shines through.
I think these young men are getting carried away with excitement at the assumption that this good looking girl is interested in them. She may actually only be interested in a potential relationship but because they're primarily seeking sex and casual sexual relationships they project that onto her. Their entire mentality is excitement and triumph at getting a "fish" caught on their hook. They want to rush the interaction along to the sex.
I presume (just speculating) that it becomes clear in the ongoing communication and dates that your DD is not looking for sex or a fwb ..but actually thinking along dating and relationship lines .... And they bail out. Because that's not what they're looking for and also because - in a weird way - they're moral enough not to want to lie to her, deceive her, use her etc when they realise she's thinking along relationship lines. I'd imagine they are also constantly trying to pull through whatever channels and they'll probably come across other girls who look like they might be up for it, so they'll change focus to them.
I can't speak for the guy she used to know who came into her workplace, but the guys approaching/getting talking in clubs ...that's usually not a place where shyer, less confident, less player type guys thrive. The opposite. So those guys are bar guys and possible fuck boys anyway.