That's like saying you buy houses from house flippers, and only at prices less than their cost. You buy stock from anybody selling it. It could be a market maker, an investor, trader, or the company itself. The other side might be realizing a gain or a loss when you do so. You don't know and it really doesn't matter. Once you do so and the trade settles, it no longer matters who you bought it from. That stock is yours and in a segregated account. If I go buy 100 shares of Intel and my trade settles, I don't have to worry about the long term financial health of who I bought it from. Think of it as the difference between buying a 100 year lease on a property and actually owning it. You might think they're functionally the same, but if the lessor doesn't pay their taxes, you'll find it's very different.
Agree, when you open an account and deposit funds at these firms, to them it is like a making a sale, they see that money as now belonging to them. A customer who actually withdraws large profits on a regular basis is actually quite shocking to them. They expect nearly all customers to lose money over the long run and keep coming back to deposit even more. Even if you make large profits on your first few trades they know based on past stats that nearly all who do so will eventually give it all back plus more.
What CFD broker do you use? I currently scalp options for stocks holding for several minutes, but I sometimes have issues with getting filled. If I use a CFD broker like IBKR, would an order of 50,000 shares of TSLA (around $10,000,000) be filled instantly? The amount is just an example, but since I scalp stocks holding them for several minutes I need instant fills. Is there a limit to how many shares can be filled quickly?