My SIL did this heinously throughout her 20s. She’s calmed down a bit (but not much) in her 30s.
I remember once there was a big summer salad in a large bowl, with slices of mango arranged on top. Maybe 8 or 9 slices. Must have been about 15 guests. She took three of the slices as well as a decent portion of the salad (oh I’m not going to be eating much meat, I prefer salad anyway). She then took a cheeseburger and a hot dog, both in buns, and a half corn on the cob. She’s tiny btw, couldn’t have weighed more than 90lbs, and eats like a bird.
All three slices of mango went in the bin - she tasted one and decided it wasn’t sweet enough. Two bites out of the burger. One out of the hot dog. It was all her mum’s cooking, and my MIL is an AMAZING cook, FIL a pro at the BBQ. Like, they regularly host 30-40 people during the holidays, never look flustered, we always bring food home, she always asks for requests and makes it. She’s and amazing home cook.
This same SIL also sees it as some sort of warped badge of honour to be profligate with single-use plastic (she gets that from her parents) and almost shows off that she doesn’t take doggy bags home. It’s so, so weird. I think she thinks it makes her look like she can afford financially not to have to worry about wastage, and make like she’s so busy and cool doing fun stuff she’d never get around to dealing with taking leftovers home / putting them in the fridge / eating them before they go off.
Anyway, I couldn’t hold it in the last time I witnessed it. Shouldn’t have but did say “look, we know you’re only interested in your own future but there are people here who are going to pay the price for our consumption habits after we’re gone. Can’t you be a bit more considerate about wasting food? Using paper towels when there’s a cloth right there? Not taking more than you’re going to eat?” I think I went on a bit because she was upset. Honestly, I don’t regret it. She was totally in the wrong.
That was probably about 10 years ago. She hasn’t changed much but she’s calmed down a bit. I just block my mind to it now. She always behaves herself when she’s at our house but it’s pretty much unchanged at her parents’ house (and she never invites us over to hers, because - as she says - she can’t cook and the take out options are better at ours).
Long story, but basically I agree OP!