The reason isn't budget per se, but also time (definitely a factor). And the fact that the Japanese entertainment industry is not union-based is also a factor. (Tsuburaya Productions is the most consistent of these companies, considering that they're chiefly a SPFX studio.)
I personally don't worry about such things. I could hardly care less if people want these films to live up to Hollywood standards, because fans tend to be worried about what America will say about these films (after decades of being called "cheesy" and "campy", etc., by cynical critics). I don't care what Mainstream America thinks. I have always enjoyed these productions, partly because America lacks the one thing the Japanese have had in spades: Imagination.
"The monster a child knows best and is most concerned with [is] the monster he feels or fears himself to be." -Bruno Bettelheim