Venture investment is limited to people who can demonstrably afford to take on the level of risk inherent to the market they're investing in. If you can't comfortably afford for the entirety of your venture investment to go to zero, you can't afford to invest in venture.
The exclusion criteria aren't perfect, (accredited investor designation is somewhat coarse-grained) but they're relatively reasonable.
The problem is access. You can’t just go to some private market, and by the time you hear about a company it’s too late to get in.
No, I expect the reason it happens this way is because it's a lot easier and simpler to take massive amounts of cash from a few investors than solicit lots of tiny investments from the general public. That and being a PLC from the get go is probably more involved than being a private limited company.
I do think ideally all companies should be public from the get go, but probably wildly impractical right now.