By the nature of it, only a few apps/products will ever be successful. The number of people/companies trying to create such apps/producs is very large today as software has the lowest possible initial investment of any kind of business (essentially zero if you have the skills already). But the number of people/companies willing to pay for some new product they don't already have is tiny. It is extremely difficult for someone to be successful in this area.

In summary: most will not make it, no matter how hard they try and even how good they are, because there's simply no room for everybody.

Success is an elusive term. For some, a successful SaaS product is one that generates $3k MRR which is achievable in many niches. The problem is, not every programmer is savvy and empathetic enough to be able to step into the shoes of a prospective customer to discover their major pain points. Instead, they tailor apps to their misaligned understanding of a problem domain instead of incrementally arriving at it through 1:1 conversation with their target audience.

I don't fully subscribe to this train of thought; I believe the pie is big enough for everyone. Now I don't have a definitive answer on why some make it and some dont, obviously, and not sure if anyone does.

It's not targeted at failed founders explicitly, however Indie Hackers is probably the community you're looking for!

https://www.indiehackers.com/

Why not simply work at a company?

I suspect that most other failed devs are indeed working at a 9-5 but looking to get the F U money from a side project and enjoy life, at least that is my situation. Also FU money is relative, I am not looking for 1B, just a small 3M nest egg is enough for me to retire.

Some people aren't made for that.

Twitter?