I think it's less entitlement, and more feeling that one child is clearly loved more than the other siblings. If family gives one child expensive gifts, cars etc, and nothing to the other children who have to struggle to get by, it's hard not to see that as favouritism in love not just support.
Like yes parents are allowed to love and spoil one child to the exclusion of others. But expecting the other children to be completely fine with that is unrealistic. And it doesn't make the unspoiled children "entitled" or bad to say "hey, this disparity feels unfair".

I can kind of relate to OP. I've received absolutely no financial help from my parents since I moved out. I paid my student loans myself and worked 3 jobs to afford rent for a room in a share house when I first moved out. Now 8 years later I've worked my way into a successful corporate sales job. I am comfortable and proud of myself for how far I've come.

I understand that I don't need financial support so I don't really resent not getting any.

However it still feels like my parents love my brothers more than me when they do things like:
- pay for their college
- pay their rent
- pay for overseas trios and living expenses
- pay for their car
- sponsor their green card to the US

At the same time, I don't blame my brothers, and I also feel that in some ways not having to rely on others has made me self reliant and more confident.

But if I was struggling financially (as I was at one time) I might resent it. And it does feel like they don't love me as much. Which doesn't feel great.