Where's the source? there are only 2,781 billionaires in worldwide, I don't think you can say such amount is "most people" when the world population is around 8 billion people. Don't exaggerate "investment" even you invest $10 Million in real estate or Bitcoin, it's still not enough to become billionaires.
As far as I understand, @OP does not say that most people are billionaires but rather most of the billionaires are created through investment. This mean many of the billionaires are rich today because of their involvement in investment ventures.
It is not impossible to become a Billionaire but you probably have about the same chances to become one as the chances to win a Lottery. Every body pretends to know what the secrets are to becoming one but then none of them are Billionaires at the end of the day.
I think it is less harder than winning a lottery. If a person have enough knowledge, influence, funds, great in financial management and a country to support his business ventures, then that person can become a billionaire one day. Being a billionaire does not depend solely on luck like winning the lottery but rather depends on the wise decision a person make.
Luck plays an important role in my opinion too, so do circumstances. Many of the Billionaires are not 'self made' even if they advertise themselves as being so. I truly believe you need to meet the right people at the right time and have the right ideas at the proper timing too. If any of us knew how to become Millionaires we would waste no more time trying other ways.
True many of the Billionaires today are not self-made but what about their ancestors that work their way in accumulating wealth? Lottery draws as far as I know only created a few billionaires unlike investments and businesses. Many people believe luck help out these self-made billionaires but I think it is their motivation, hardworks, knacks, smart decision and influence(and the blessings and guidance by the Powerful One for religious people).