How common is unpaid intern work?
Why should big, wealthy companies expect young people to work for free?
Is this a deliberate form of gate keeping so that only people from wealthy families can get a foot in the door?
On the contrary in the US, I often see young people take jobs which pay around minimum wage and will not count as industry experience in their field - when they have enough qualifications to execute usefully on much higher paying jobs (even if the employer would still be underpaying them compared to the usual hires). My business would have used them - but couldn't find them easily. The job market is a mess.
However, previous governments set up ""welfare-to-work"" schemes, effectively a government subsidy to employers, where people were required to work for free: https://www.recruiter.co.uk/news/2012/03/no-end-controversy-...
(absurdly huge controversy, the government tried to legislate the outcome of this specific lawsuit through retroactive legislation, in violation of ECHR https://ukhumanrightsblog.com/2014/07/08/retrospective-legis... )
Heck it is even hard to get companies to onboard experienced developers with no experience in the stack already used in the company.
Most companies don't expect young people to work for free, they just don't hire them. What you are left is companies who expect young people to work for free, which obviously are not good companies.
It has always been the case that new hire are a net negative to start with. Even experienced people are probably a net negative for a few weeks. Inexperienced people are more of a long term investment.
The law requires employers to pay minimum wage to anyone who is a worker (i.e. is doing work for the company) - but "sitting in" doesn't have to be paid. Exceptions are also made for volunteering, and placements as part of a university course (these can be unpaid).
Enforcement isn't consistently applied so some smaller outfits get away with it. The big FAANGs and other major companies pay.