because 5rax will literally be at your doorstep with 24ish mnm by the time your mutas pop at 715-730
you can get away with 16g but it's weaker/suboptimal. modern 3h muta goes 13h 12g, saviors 16g hasn't been used in like a decade at this point
3h lurker is fine until you get pretty high on the ladder. you can ignore muta builds for a while to simplify gameplay