[extrapickles]

You could in theory do that. The downsides are that a turbopump needs comparatively perfect fuel, requires liquid oxygen instead of air, and would be inefficient in non-rocket applications (the ‘waste’ has other uses like cooling/protecting the rocket nozzle in some rockets).

Think of it as an engineering tradeoff where you get to pick 2 things to optimize for: weight, efficiency and size. A turbopump (and preburner) optimizes for weight/size. A ships engine picks efficiency twice, making it massive and heavy.