Steam isn't a monopoly. I, and everyone I know who uses steam is familiar with GOG or Epic games or Battle.net or some other service. You can even distribute your game independently (e.g. in the case of minecraft and some of the most successful PC games of all time) or just distribute it as a web game (increasingly feasible as WebGL, WebGPU, WASM etc. continue to advance).

Steam is successful because it has good user experience compared to alternatives, and has a lot of major titles.