Not everybody has aptitudes for STEM fields. Furthermore, I don't understand how this impacts STEM at all. Are you trying to imply that other subject areas don't have actual academic rigor?
While it's true that "not everyone has the aptitude for it" (that'd be technically true for any profession), quite a few people have the aptitude for it, they just choose something else that's not as viable.
For instance, per the NCES data I posted earlier.
Just a few examples:
78k degrees conferred in Computer Science
88k conferred in Performing Arts
I'd say that higher percentage of people have the intelligence/aptitude/abilities required to get a decent job as a software developer as opposed to being a successful actor or a famous singer.
And you can definitely see in the charts where the patterns have been shifting over the past few decades.
The number of people going for things like performance arts and journalism exploded in the 90's.
Part of it as a supply/demand issue (just about every company has a need for things like IT people, Accounting/Finance, Marketing/Sales), gainful careers in the performing arts and journalism are much harder to come by.
The other part is that those fields are very much a "feast for famine" environment, there's not as many tiers or levels of those jobs where a decent living can still be achieved.
To sort of highlight what I mean by that...
Take someone who's in the field of accounting/finance/economics. One doesn't have to be a Janet Yellen/Warren Buffett/Paul Krugman level person to still make a decent living in that field. Those uber-high paying opportunities exist, but there are plenty of "rungs on the ladder" for people who aren't quite at their level to still make a good living.
If you're an economics major who's in the 80th percentile in terms of skill/aptitude, you may not be able to be the next Secretary of the Treasury or CEO of a Wall Street investment firm, but you can still be a CPA, hold a higher level position in the accounting department of a private company, etc...
If you're a tech person who's in the 80th percentile, you may never be the next Chief Technology Officer for Microsoft or Apple, but you could still be a database administrator
The same isn't true for performing arts, there aren't a lot of "mid-level" positions in those fields. It's very much a "you're either the absolute best of the best, and wildly successful" or "you're a starving artist" dichotomy.
I think since the mid-90's, people have gotten some unrealistic expectations, and as a result, have tried to gravitate toward the "sexier" "flashy" professions.