The thrust to weight ratio of a cluster of rocket engines is not a linear relationship. Adding one more engine might add N+1 thrust but it also adds at least N+

The thrust to weight ratio of a cluster of rocket engines is not a linear relationship. Adding one more engine might add N+1 thrust but it also adds at least N+1 mass. So you have more thrust but now more mass to lift and a worse mass ratio (the ratio of fuel to non-fuel). To scale your engine cluster you want to add an engine worth of thrust but not an engine worth of mass. To that end you reduce the mass of your piping or use one pump for multiple engines. All those changes require engineering and testing because the dynamics of the system have changed. For instance a fuel pipe of X thickness and Y material may have some weird failure mode if you make it just a little thinner to save mass.