Supplying capital is compensated by the market because it's a scarce factor of production. It is a scarce factor of production because (i) there are risks associated with its provision, (ii) there's an opportunity cost. Nobody would invest in a startup if there wasn't an expectation of an ROI. Because it's risky, that ROI is high. The ROI is high because the cost base is deflated by the market. This is a risk premium, and it's built into the cost base by supply and demand. This basic mechanism underpins all capital markets and is the reason capital markets function at all.
Your statement about the scalability of capital is of course true. Capital offers the owner intense amounts of leverage and scale. The same can in general not be said about labor except in domains where that labor is scaled by technology (e.g. dev), media (e.g. instagram influencer), or being in a managerial position (e.g. decisions impact 1000-person org), which probably covers about 2% of the workforce.