Suppose there are 3 people involved in a startup: the founder, the engineer and the investor. The engineer gets $300,000 for 2 years' work. The founder gets $200,000 for 3 years' very intense work + $10 million if the startup succeeds. The investor puts in capital, does no work, and gets $40 million if the startup succeeds.
It is arguable that the founder and the engineer are getting a comparable amount but with a different risk profile. The socialist argument is that the person who only provides capital is overpaid for doing that. Whether that argument has an economic justification, or just a political or moral justification, and whether you believe that argument, is another matter. But it is clear that, if the investor got less, there would be more to pay both of the people who contribute work.
Perhaps a source of confusion is that successful founders often go on to become investors with the money they have collected, for example Paul Graham or Marc Andreessen. You could also argue that someone like Mark Zuckerberg, while still nominally 'working' as CEO, has gone from being a founder in Facebook the product to being an investor who provides capital for various Facebook the company projects, and that perhaps much of his wealth was accumulated in the second stage.
imo market forces have some equilibrium in current state
Its not like founders are forced to. They can always be profitable and fund from their own operational cash flow