the fact is that the japanese had already back in 1987 the tools that makes things easier for you today. They not only had 15 years ahead from us, but they also had superior hardware and superior tools.
When you look at a japanese shmup, the guy had already the ability to check the sprites patterns on screen ! He could then tweak it, modifiy it, and check it right away on his screen XD !
When in europe the coder had to modify the thing and generate a new assembled program each time a modification was done, the japanese did what he wanted on screen, outputed the new pattern, and the coder just had to incorporate those in the code, and then assemble it.