Most folks simply don't have the resources or knowledge to do this, particularly the testing part. I agree there are some places where diminishing returns certainly apply (the strut rod hole reinforcement probably is a good example, especially with rubber bushings)... but eliminating flex in structural components is nearly always a plus. A ton of spot welds would certainly approach seam welding, but I don't know of a "hobby" level spot welder that is capable of doing the job, so again, out of reach of the average hobbyist. Plenty have a MIG welder though.
Also, I know and work with a lot of engineers... and while that's the textbook method, it gets used less often than you might think.