I cannot answer either the more broad or more specific questions in these threads. But my 2 cents as to:
"Why do we hire reactjs programmers with e-commerce experience instead of react programmers, e-commerce programmers, or programmers?"
In my neck of the woods, we generally hire specifically PeopleSoft Programmers, ideally with Payroll experience. Here's why:
1. First and foremost, developers need to do more than spit out code. They need to thoroughly understand business problems, work with business analysts and product owners to hash out requirements, be able to meaningfully discuss these requirements, create design that they can independently sanity-check meets these requirements, and then work on translating it into code.
2. They need PeopleSoft experience and knowledge - it's a specific ecosystem and language with specific requirements and constraints and capabilities. Programmer needs to fully understand these early on when they discuss requirements and solution with business.
3. Payroll experience helps, again for them to be able to discuss things meaningfully with the business.
I know that this ("Dev needs to be able to gather requirements") is self-evidently true across development world, but I want to underline that our developers can meaningfully discuss intricacies of Canadian and Provincial tax code, payroll processes and regulations, time and labour, benefits, pension, and other adjustments with their functional counterparts; while also having meaningful discussions with our DBA & Infra teams on indexes, stats, etc. All in the specific world of PeopleSoft where code resides in database, with specific component processor which offers about 30 different ways to trigger code on a page, while ensuring data integrity etc.
With that in mind, historically, it's just as easy and perhaps easier/faster/cheaper, to train a new grad (and we do, a lot!), rather than get somebody experienced in a completely different ecosystem and try to retrain them.
Now, PeopleSoft is a very specific (quirky, legacy, idiosyncratic, crazy, etc) ecosystem, but I think using it as an extreme example may show some of the core reasons why even in less extreme environments, requirements are fairly specific. Yes, a good programmer with wide experience has a lot of skills. But by numbers, I think most devs work in business areas with specific set of common industry/sector problems and solutions, which reward specialization.