My theory: in the United States, speaking multiple languages can be a sign of coming from a more financially sound household. I wouldn’t expect a second language to be any better for the brain than studying math or learning to play an instrument. As always, the most important factor is money.
I would be surprised if the majority of bilingual speakers in the US aren't first/second gen immigrants. Not exactly known to be "a sign of coming from" wealth
I think this it. In Michigan public schools at least there are mandatory language courses for 2 years but these do not get most people anywhere close to fluency. Apparently I'm qualified enough to get a 4 on AP German but at my peak I could only get about 60% of your typical Der Spiegel headline. Everyone I know that knows another language at at least roughly a C1 level knows it because of family. But I imagine there are some people with tutors or better schools who start early with a language and probably end up pretty good at it.
Second generation immigrants struggle to speak the language of their parents, however. This happens at least with immigrants from my country to the U.S.
India for instance -- A typical kid in many places here (especially southern part of the country) has 3 languages formally taught in school and consumed as popular media daily.
Having privilege in these regions usually translates to levels of literacy like having reading ability (in addition to just speaking for trade), reading literature, speaking fluency, broader vocabulary, "better" accent, etc. -- all of which, in thousand different ways, signal one's socio-economic status in every interaction.
On the finance part, I think it's complex, in particular relative to who.
For instance, immigrants will massively be bilingual in general. We could argue there are in a better financial and educational position than those who couldn't immigrate in the first place, but then relative to the country they live, they can be on bottom of the social ladder.
On the parrallel with studying other subjects, I think it rings true, the only difference would be that bilingualism can be "forced" upon children by circumstances or environment, where math or instruments usually come later and/or on a choice basis.
