Why Steam can be considered a monopolistic platform?

For them to be prosecuted as a monopoly, or be considered one legally, it would have to be shown that they achieved or maintain their dominant market position by preventing or undermining competition. Say by having a bunch of exclusivity deals to keep big name titles only on their storefront, or by buying out any competitor that gained traction.

Monopoly isn’t about being the biggest seller in a market, it’s about being the biggest player in the market by undermining competition and restricting commerce.

Edit: want to clarify there is a distinction between the legal meaning of monopoly (see the Sherman anti trust act and other subsequent laws and rulings) and the colloquial usage (Only seller in the market). Steam is nether.