I don't even think it's about parents 'putting kids first' or giving into their every want. A lot of it comes down to parents who are insecure or who get their self worth from their children and nothing else. What I mean by that is they can't feel good about anything they do unless it's constantly pleasing their children. My mum is one of these. It took having children of my own to see how she is with them and realise that explained a lot of my childhood! She literally feels distressed to the point of tears or raging anger (like walking around my house slamming my doors) if I have my daughter sit out for doing something or if she has a meltdown because I won't let her have something she wants. I'm really strict in the sense that my children are the most important thing to me and as you say their needs always come first. But I don't give in to every want (in fact, probably not most of them) because I think learning to deal with disappointment and frustration is really important.
My mum though is the opposite. I truly cannot remember a time in my childhood when she didn't give me something I asked for. If we went into a pet shop and I wanted a puppy, I got a puppy. I wanted a horse, I got a horse. I wanted to go out for dinner when she'd already cooked something, we did it. I wanted sweets for breakfast, she'd bring them to me in bed (literally, I'm not making that up, I pretty much ate sweets in bed for breakfast for about a year, no wonder I have no many dental issues now!). It's because psychologically her self worth was completely tied up in whether I was content or not. I think because she was a single parent she felt she had to overcompensate (she didn't, she did a perfectly fine job without having to buy me all that stuff). But she couldn't cope with me being upset if she said no, so she never said no.
When she last visited, my daughter wanted to take her shoes off in the car. I said I'd prefer if she didn't, but if she did, she had to put them on herself when we got to where we were going because I wasn't doing it. She said she would. When we got there, she didn't want to. So I told her we weren't going anywhere until she put her shoes on like she'd promised. My mum wanted to do it for her, but I had to stop her and said she could do it herself and we'd wait. My mum literally ran into the store crying because I made my daughter put her shoes on (she's school age, she can put on her own shoes, they're velcro). But she just can't cope with seeing anyone do anything they don't want to do or saying no.
I think you're right there is a confusion about needs and wants, and some of it is a bit of wanting to keep up with what everyone else is doing or buying. But I also think it's probably due to low self esteem and self worth whose happiness and confidence is tied into whether or not their kids seem happy all the time. For my mum, any sign of a tantrum or feeling upset on my part meant she was a failure, so she tried to avoid it at all costs, and I'm quite certain there are still a lot of people who have that mentality.