It’s not Defold’s design choice, it’s Lua’s (and many other languages):
Alternatively, you can check out the Native Extension with PCG random generation (notice, it also has a .seed() function):
Or “Defold-RNG” with both PCG and MT:
Or “Dicebag” module (also with .set_up_rng() function):
Regarding one case, where seed is used for generating worlds - yeah, it comes to my mind immediately: 
As you can see, if no seed is provided the default is “random”, so I get your confusion and I believe it could be done in the very same way, but in case of pseudo-random generators it was always on developer to decide which seed to provide, the program doesn’t assume it for us, it’s weird, I agree